Optical spectroscopy of (candidate) ultra-compact X-ray binaries
Gijs Nelemans
We present (preliminary) results of our systematic spectroscopic study of (candidate) ultra-compact X-ray binaries. Most candidates are confirmed and we found the first optical spectra of (pure) carbon-oxygen accretion discs.
aipcheck.tex
%
% $Id: aipcheck.tex,v 1.7 2002/01/11 19:50:46 frank Exp $
%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Testing for potential problems with this class
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\newif\ifproblem
\newif\iftimesok

\makeatletter
\def\IfStandaloneCheck{\def\next{aipcheck}
  \edef\currjob{\jobname}
  \edef\next{\meaning\next}
  \edef\currjob{\meaning\currjob}
  \ifx\currjob\next
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\makeatother

\typeout{***********************************************}
\typeout{*}
\typeout{* Testing if all files required for the aipproc}
\typeout{* class are available ...}
\typeout{*}
\typeout{***********************************************}

\typeout{*}
\typeout{* Looking for LaTeX2e ... }
\ifx\documentclass\undefined
 \typeout{*}
 \typeout{* Sorry this is a fatal error:}
 \typeout{*}
 \typeout{* The aipproc class can only be used with LaTeX2e which is}
 \typeout{* the standard LaTeX since 1994!}
 \typeout{*}
 \typeout{* Please make sure that your version of LaTeX is up-to-date}
 \typeout{* before attempting to use this class.}
 \typeout{*}
 \expandafter\stop
\else
 \typeout{* ... ok }
\fi


\def\next#1/#2/#3\next{#1#2}
\typeout{*}
\typeout{* Testing that LaTeX2e is not too old ... }
\ifnum\expandafter\next\fmtversion\next<199612 \relax
 \typeout{* ... what a vintage! }
 \typeout{*}
 \typeout{* Sorry this is a fatal error:}
 \typeout{*}
 \typeout{* The aipproc class can only be used with a recent version}
 \typeout{* of LaTeX2e. Your version is dated \fmtversion\space --- but}
 \typeout{* at least the 1996/12/01 version is required!}
 \typeout{*}
 \typeout{* Please make sure that your version of LaTeX is up-to-date}
 \typeout{* before attempting to use this class.}
 \typeout{*}
 \expandafter\stop
\else
 \ifnum\expandafter\next\fmtversion\next<199806 \relax
   \typeout{* ... probably ok }
   \typeout{*}
   \typeout{* Your version of LaTeX2e is quite old --- the aipproc class}
   \typeout{* hasn't been tested with your release.}
   \typeout{*}
   \typeout{* We believe that it will probably work, but if you encounter}
   \typeout{* problems you will need upgrade your installation.}
   \typeout{*}
   \typein{* Type <return> to continue ...}
   \problemtrue
 \else
   \typeout{* ... ok }
 \fi
\fi


\typeout{*}
\typeout{* Looking for aipproc.cls ... }
\IfFileExists{aipproc.cls}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* Before you can use the aipproc class you have to unpack}
     \typeout{* it from the documented source.}
     \typeout{*}
     \typeout{* Run LaTeX on the file 'aipproc.ins', e.g.,}
     \typeout{*}
     \typeout{* \space\space latex aipproc.ins}
     \typeout{*}
     \typeout{* or whatever is necessary on your installation to process}
     \typeout{* a file with LaTeX. This should unpack a number of files for you:}
     \typeout{*}
     \typeout{* aipproc.cls \space and \space aip-*.clo}
     \typeout{*}
     \typeout{* After that retry processing this guide.}
     \typeout{*}
     \stop
}

\typeout{*}
\typeout{* Looking for aipxfm.sty ... }
\IfFileExists{aipxfm.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aipxfm.sty file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}

\typeout{*}
\typeout{* Looking for aip-8s.clo ... }
\IfFileExists{aip-8s.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-8s.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}

\typeout{*}
\typeout{* Looking for aip-8d.clo ... }
\IfFileExists{aip-8d.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-8d.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}


\typeout{*}
\typeout{* Looking for aip-6s.clo ... }
\IfFileExists{aip-6s.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-6s.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}


\typeout{*}
\typeout{* Looking for aip-arlo.clo ... }
\IfFileExists{aip-arlo.clo}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The aip-arlo.clo file which is part of the aipproc distribution}
     \typeout{* must be installed in a directory which is searched by LaTeX.}
     \typeout{*}
     \typeout{* Please install this file and retry.}
     \typeout{*}
     \stop
}

\typeout{*}
\typeout{* Looking for fixltx2e.sty ... }
\IfFileExists{fixltx2e.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found, trying fix2col.sty instead ... }
     \typeout{*}
     \IfFileExists{fix2col.sty}
	 {
	  \typeout{* ... ok }
	 }
	 {
	  \typeout{* ... not found! }
	  \typeout{*}
	  \typeout{* Sorry this is a fatal error:}
	  \typeout{*}
	  \typeout{* Your LaTeX distribution contains neither fixltx2e.sty}
	  \typeout{* nor fix2col.sty.}
	  \typeout{*}
	  \typeout{* This means that it is either too old or incompletely}
	  \typeout{* installed.}
	  \typeout{*}
	  \typeout{* fixltx2e.sty is part of the standard LaTeX distribution}
	  \typeout{* since 1999; fix2col.sty is an earlier version of this}
	  \typeout{* package.}
	  \typeout{*}
	  \typeout{* Best solution is to get the latest LaTeX distribution.}
	  \typeout{* If this is impossible for you, download fix2col.sty.}
	  \typeout{* You can get this software from a CTAN host.}
          \typeout{* Refer to http://www.ctan.org and search for "fix2col".}
	  \typeout{*}
	  \typeout{* After you have updated your LaTeX distribution}
	  \typeout{* retry processing this guide.}
	  \stop
     }
}

\typeout{*}
\typeout{* Looking for fontenc.sty ... }
\IfFileExists{fontenc.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The fontenc package, which is part of standard LaTeX}
     \typeout{* (base distribution) has to be installed at the site to}
     \typeout{* run the aipproc class.}
     \typeout{*}
     \typeout{* The fact that it cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* improperly.}
     \typeout{*}
     \typeout{* Please make sure that your version of LaTeX is okay}
     \typeout{* before attempting to use this class. The LaTeX distribution}
     \typeout{* contains the file "ltxcheck.tex" which can be used to}
     \typeout{* test the basic functionality and integrity of your installation.}
     \typeout{*}
     \stop
    }

\typeout{*}
\typeout{* Looking for calc.sty ... }
\IfFileExists{calc.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Sorry this is a fatal error:}
     \typeout{*}
     \typeout{* The calc package, which is part of standard LaTeX}
     \typeout{* (tool distribution) has to be installed at the site}
     \typeout{* to run the aipproc class.}
     \typeout{*}
     \typeout{* The fact that it cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts.}
     \typeout{*}
     \typeout{* Please make sure that the tools distribution of LaTeX}
     \typeout{* is installed before attempting to use this class.}
     \typeout{*}
     \typeout{* (You might be able to get calc.sty separately for your}
     \typeout{* installation if you are unable to upgrade to a recent}
     \typeout{* distribution for some reason.)}
     \typeout{*}
     \stop
    }

\typeout{*}
\typeout{* Looking for varioref.sty ... }
\IfFileExists{varioref.sty}
    {
     \typeout{* ... ok }
     \gdef\variorefoptionifavailable{varioref,}
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The varioref package, which is part of standard LaTeX}
     \typeout{* (tool distribution) is not installed at this site.}
     \typeout{*}
     \typeout{* The fact that it cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts.}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you cannot make use of the options "varioref" or "nonvarioref".}
     \typeout{*}
     \typeout{* Please also note that the aipguide.tex documentation}
     \typeout{* normally uses the "varioref" option to show its}
     \typeout{* effects (which  will now fail).}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
     \gdef\variorefoptionifavailable{}
     \let\vpageref\pageref
     \let\vref\ref
    }

\typeout{*}
\typeout{* Looking for times.sty ... }
\IfFileExists{times.sty}
    {
     \begingroup
% load times and forget it immediately again
       \RequirePackage{times}
       \global\expandafter\let\csname ver@times.sty\endcsname\relax    
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       \ifx\rmdefault\next
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         \endgroup
         \timesoktrue
       \else
         \endgroup
     \typeout{* ... obsolete! }
     \typeout{*}
     \typeout{* Serious problem detected:}
     \typeout{*}
     \typeout{* The times package, which is part of standard LaTeX}
     \typeout{* (psnfss distribution) is obsolete at this site.}
     \typeout{*}
     \typeout{* The fact that it contains incorrect code either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts with old files remaining!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but}
     \typeout{* you have to specify the option "cmfonts" which result in}
     \typeout{* documents which are not conforming to the AIP layout specification!}
     \typeout{*}
     \typeout{* You can also try using the class in the following way:}
     \typeout{*}
     \typeout{* \space\space \string\documentclass[cmfonts]{aipproc}}
     \typeout{* \space\space \string\usepackage{times}}
     \typeout{* \space\space ...}
     \typeout{*}
     \typeout{* With luck this will result in Times Roman output but chances}
     \typeout{* are that you will get a larger number of error messages in}
     \typeout{* which case you have to remove the \string\usepackage declaration.}
     \typeout{*}
     \typein{* Type <return> to continue ...}
          \problemtrue
          \gdef\psnfssproblemoption{cmfonts}
          \def\textdegree{$^\circ$}            % used below but now
                                               % not setup
       \fi
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Serious problem detected:}
     \typeout{*}
     \typeout{* The times package, which is part of standard LaTeX}
     \typeout{* (psnfss distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you have to specify the option "cmfonts" which result in}
     \typeout{* documents which are not conforming to the AIP layout specification!}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
     \gdef\psnfssproblemoption{cmfonts,}
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\iftimesok % don't bother testing other font options if times already
           % bad

\typeout{*}
\typeout{* Looking for t1ptm.fd or T1ptm.fd ... }
\IfFileExists{t1ptm.fd}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found, trying T1ptm.fd ... }
     \IfFileExists{T1ptm.fd}
          {
           \typeout{* ... ok }
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          {
           \typeout{* ... not found}
           \typeout{* Serious problem detected:}
           \typeout{*}
           \typeout{* The times package, which is part of standard LaTeX}
           \typeout{* (psnfss distribution) is available but the corresponding}
           \typeout{* .fd file (defining how to load Times Roman) is missing.}
           \typeout{*}
           \typeout{* The fact that this package is only partially installed}
           \typeout{* means that you LaTeX installation is unable to use Times}
           \typeout{* Roman fonts!}
           \typeout{*}
           \typeout{* You can use the aipproc class without this package but }
           \typeout{* you have to specify the option "cmfonts" which result in}
           \typeout{* documents which are not conforming to the AIP layout}
           \typeout{* specification!}
           \typeout{*}
           \typein{* Type <return> to continue ...}
           \problemtrue
           \timesokfalse
           \gdef\psnfssproblemoption{cmfonts,}
          }
    }

\fi


\newcommand\CheckFDFile[3]{%
  \typeout{*}
  \typeout{* Looking for #1#3.fd or #2#3.fd ... }
  \IfFileExists{#1#3.fd}
    {
     \typeout{* ... ok }
    }
    {
     \IfFileExists{#2#3.fd}
      {
       \typeout{* ... ok }
      }
      {\problemtrue
       \typeout{* ... not found! }
      }
    }
}

\iftimesok % don't bother testing other font options if Times already bad

%\CheckFDFile{ot1}{OT1}{ot1ztmcm}
%\CheckFDFile{oml}{OML}{omlztmcm}
%\CheckFDFile{oms}{OMS}{omsztmcm}
%\CheckFDFile{omx}{OMX}{omxztmcm}

\typeout{*}
\typeout{* Looking for mathptm.sty ... }
\IfFileExists{mathptm.sty}
    {
     \typeout{* ... ok }
     \CheckFDFile{ot1}{OT1}{ptmcm}
     \CheckFDFile{oml}{OML}{ptmcm}
     \CheckFDFile{oms}{OMS}{pzccm}
     \CheckFDFile{omx}{OMX}{psycm}
     \ifproblem
      \typeout{*}
      \typeout{* Problem detected:}
      \typeout{*}
      \typeout{* The mathptm package, which is part of standard LaTeX}
      \typeout{* (psnfss distribution) was found but some or all of its}
      \typeout{* support files describing which fonts to load are missing!}
      \typeout{*}
      \typeout{*}
      \typeout{* The fact that this package is only partially installed}
      \typeout{* means that the mathptm package cannot be used!}
      \typeout{*}
      \typeout{* You can use the aipproc class without this package but }
      \typeout{* you have to specify the option "nomathfonts" so that}
      \typeout{* math formulas will be typeset using Computer Modern.}
      \typeout{*}
      \typein{* Type <return> to continue ...}
      \problemtrue
      \gdef\psnfssproblemoption{nomathfonts,}
     \else
      \typeout{*}
      \typeout{* Looking for mathptmx.sty ... }
      \IfFileExists{mathptmx.sty}
       {
        \typeout{* ... ok }
        \CheckFDFile{ot1}{OT1}{ztmcm}
        \CheckFDFile{oml}{OML}{ztmcm}
        \CheckFDFile{oms}{OMS}{ztmcm}
        \CheckFDFile{omx}{OMX}{ztmcm}
        \ifproblem
	  \typeout{*}
	  \typeout{* Problem detected:}
	  \typeout{*}
	  \typeout{* The mathptmx package, which is part of standard LaTeX}
	  \typeout{* (psnfss distribution) was found but some or all of its}
	  \typeout{* support files describing which fonts to load are missing!}
	  \typeout{*}
	  \typeout{*}
	  \typeout{* The fact that this package is only partially installed}
	  \typeout{* means that the mathptmx package cannot be used!}
	  \typeout{*}
	  \typeout{* You can use the aipproc class without this package but }
	  \typeout{* you have to specify the option "mathptm" (no x) so that}
	  \typeout{* math formulas use the older version with upright greek letters.}
	  \typeout{*}
	  \typein{* Type <return> to continue ...}
	  \problemtrue
	  \gdef\psnfssproblemoption{mathptm,}
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       }
       {
	\typeout{* ... not found! }
	\typeout{*}
	\typeout{* Problem detected:}
	\typeout{*}
	\typeout{* The mathptmx package, which is part of standard LaTeX}
	\typeout{* (psnfss distribution) can not be found.}
	\typeout{*}
	\typeout{* This is unfortunate but not a disaster as the older}
	\typeout{* version of the package "mathptm" (no x) seems to exist.}
	\typeout{*}
	\typeout{* You can use the aipproc class without this package but }
	\typeout{* you have to specify the option "mathptm" so that}
	\typeout{* math formulas use the older version with upright greek letters.}
	\typeout{*}
	\typein{* Type <return> to continue ...}
	\problemtrue
	\gdef\psnfssproblemoption{mathptm,}
       }
      \fi
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The mathptm package, which is part of standard LaTeX}
     \typeout{* (psnfss distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you have to specify the option "nomathfonts" so that}
     \typeout{* math formulas will be typeset using Computer Modern.}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
     \gdef\psnfssproblemoption{nomathfonts,}
    }

\typeout{*}
\typeout{* Looking for mathtime.sty ... }
\IfFileExists{mathtime.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* The mathime package can not be found.}
     \typeout{*}
     \typeout{* This is not a serious problem because this package is}
     \typeout{* only of interest if you own the commerical MathTime fonts.}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you cannot use the "mathtime" option of the class.}
     \typeout{*}
     \typein{* Type <return> to continue ...}
     \problemtrue
    }
\else
\fi % iftimesok

\typeout{*}
\typeout{* Looking for graphicx.sty ... }
\IfFileExists{graphicx.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The graphics package, which is part of standard LaTeX}
     \typeout{* (graphics distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you cannot use commands like \protect\includegraphics
                or \protect\resizebox}
     \typeout{* in this case.}
     \typeout{*}
     \typeout{* Please note that you will get a further error message below}
     \typeout{* about: "graphicx.sty not found" because the class will try}
     \typeout{* to load this package! Type return in response to that error.}
     \typeout{*}
     \typeout{* As a result the illustrations in aipguide will look strange.}
     \typeout{*}
     \typein{* Type <return> to continue ...}

     \gdef\resizebox##1##2{}
     \gdef\includegraphics{\textbf{graphics package missing:}}
     \problemtrue
    }

\typeout{*}
\typeout{* Looking for textcomp.sty ... }
\IfFileExists{textcomp.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The textcomp package, which is part of standard LaTeX}
     \typeout{* (base distribution) can not be found.}
     \typeout{*}
     \typeout{* The fact that this package cannot be found either means that}
     \typeout{* this LaTeX release is too old or that it was installed}
     \typeout{* only in parts!}
     \typeout{*}
     \typeout{* You can use the aipproc class without this package but }
     \typeout{* you will always get the error: "textcomp.sty not found"}
     \typeout{* because the class will try to load this package!}
     \typeout{* Type return in response to that error.}
     \typeout{*}
     \typein{* Type <return> to continue ...}

     \def\textdegree{$^\circ$}         % used below but now
                                       % not set up
     \problemtrue
    }


\typeout{*}
\typeout{* Looking for url.sty ... }
\IfFileExists{url.sty}
    {
     \typeout{* ... ok }
    }
    {
     \typeout{* ... not found! }
     \typeout{*}
     \typeout{* Problem detected:}
     \typeout{*}
     \typeout{* The url package, which should be part of a good LaTeX}
     \typeout{* distribution, can not be found.}
     \typeout{*}
     \typeout{* Without this package you will not be able to use the \string\url}
     \typeout{* command. Try to download this package from a CTAN  host.}
     \typeout{* Refer to http://www.ctan.org and search for "url".}
     \typeout{*}
     \typein{* Type <return> to continue ...}

     \problemtrue
    }


\makeatletter

\typeout{*}
\typeout{* Looking for natbib.sty ... }
\IfFileExists{natbib.sty}
    {
     \IfStandaloneCheck
       {\begingroup
        \let\@listi\relax
        \let\thebibliography\@empty
        \let\bibstyle\@empty
        \RequirePackage{natbib}
        \@ifpackagelater{natbib}{1999/05/29}
	  {
           \typeout{* ... ok }
	  }{
           \typeout{* ... might be too old! }
           \typeout{*}
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nelemans.tex
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\begin{document}

\title{ Optical spectroscopy of (candidate) ultra-compact X-ray binaries}

\classification{95.75.Fg, 97.10.Cv, 97.10.Gz, 97.80.Jp}

\keywords{X-ray binaries -- optical observations}

\author{Gijs Nelemans}{
  address={Institute of Astronomy, Cambridge, UK \\and\\
Department of Astrophysics, Radboud University Nijmegen, The Netherlands}
}

\author{Peter Jonker}{
  address={Harvard-Smithsonian Center for Astrophysics, Cambridge, MA,
  USA}
}


\begin{abstract}
We present (preliminary) results of our systematic spectroscopic study
of (candidate) ultra-compact X-ray binaries. Most candidates are
confirmed and we found the first optical spectra of (pure)
carbon-oxygen accretion discs.
\end{abstract}

\maketitle

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\section{Introduction}

Ultra-compact X-ray binaries (UCXBs) are close binaries with periods
less than about one hour, in which a neutron star (or possibly a black
hole) accretes material from a companion star. Their short periods
rule out ordinary hydrogen-rich companion stars, since these stars are
too big; they do not fit in the Roche lobe \citep[e.g.][]{nrj86}

In recent years a renewed interest has developed in these systems for
a number of reasons. Improved observing facilities (e.g. large optical
telescopes and sensitive X-ray satellites) make it possible to study
them in more detail. But more importantly the number of known
systems has increased. In particular the discovery of three transient
UCXBs \citep[e.g.][]{mss+02,rss02,mjs03} in which the millisecond
pulsations of the accreting neutron star were seen, has been an
exciting development.

We have started a systematic spectroscopic study of known and
candidate UCXBs (excluding systems in globular clusters) in order to
confirm/reject the candidates, to constrain possible formation
scenarios and to open the way to study the chemically peculiar
accretion process that is expected to operate in these binaries.


\section{Current open questions}


The formation of UCXBs has been proposed through three channels, in
which the donor stars are either white dwarfs, helium stars, or the
remnants of evolved main sequence stars \citep[see][and references
therein]{prp02}. However, there are still many open questions about
UCXBs. One of the first priorities is to increase the number of known
systems and to find their orbital periods. Besides the orbital period
there are a number of properties that indicate that a particular
system could be an UCXB.  \Citet{vm94} studied the absolute magnitudes
of X-ray binaries and derived a relation between the absolute
magnitude, the orbital period and the X-ray luminosity, based on the
assumption that the absolute magnitude is dominated by the irradiated
accretion disc, whose surface is determined by the size of the binary,
i.e. the orbital period. They empirically gauged this
relation. Systems with absolute magnitudes fainter than about $M_V =
4$ have a good chance to be UCXBs.

\begin{table}
\begin{tabular}{llllc}
\hline \hline Name & Period & Ne & M$_{\rm V}$& confirmed? \\
 & (min) & & & \\ \hline \hline 
4U 1543-624 & 18$^a$  & y & & $\checkmark$ \\ 
4U 0614+09 & ?  & y & 5.4 & $\checkmark$ \\ 
2S 0918-549 & ?  & y & 6.9 & ($\checkmark$) \\ 
4U 1822-00 & ?  & & 5.5 &  ($\checkmark$) \\ 
4U 1556-605 & ? & y & & $\times$\\ 
XB 1905+000 & ? & & 4.9 & ??\\  \hline \hline
$a$ \citet{wc04}
\end{tabular}
\caption{Overview of the UCXB candidates.}
\label{tab:candidates}
\end{table}


\citet{jpc00} found features in the X-ray spectrum of the 20 min UCXB
4U 1850-087, which they attributed to enhanced Ne in the system and
found similar features in 3 other systems, making them good UCXB
candidates. In Table~\ref{tab:candidates} we list our selection of
candidates, based either on their absolute magnitudes or the the
precence of the ``Ne feature''.

Optical spectroscopy of UCXBs might also be a good way to study the
formation of UCXBs, as the different formation scenarios will lead to
different chemical composition of the transferred material and thus of
the accretion disc. The first constraints on the chemical composition
have come from the properties of the type I X-ray bursts observed from
the 11 min globular cluster sources 4U 1820-30, suggesting helium rich
material \citep{bil95}.  In the following section we will describe the
results of our observations, which are already summarized in the last
column of the table.


\section{Optical spectroscopy: results}

\subsection{The VLT and FORS2}

We used the FORS2 spectrograph on the 8.2m Very Large Telescope (VLT)
of the European Southern Observatory at Paranal to obtain optical
spectra of our candidate UCXBs. The observations were taken in the
spring of 2003 and 2004. In 2003 we used the 1400V and 600RI
holographic grisms, with a 1" slit, using 2x2 on-chip binning.  This
setup resulted in coverage of 4620 -- 5930 \AA\ with mean dispersion
of 0.64 \AA/pix for the 1400V spectra and 5290 -- 8620 \AA\ with mean
dispersion of 1.63 \AA/pix for the 600RI spectra. The 2004 spectra are
taken with the 600B and 600RI grisms. The 600B grism covers the range
3325 -- 6367\AA\ with dispersion of 1.48 \AA/pix. All spectra were
reduced using standard IRAF tasks.


\subsection{4U 0614+09, 4U 1543-624 and 2S 0918-549}

\begin{figure}
  \includegraphics[angle=-90,width=\textwidth]{nelemans_f1}
  \caption{VLT spectra of 4U 0614+09, 4U 1543-624 and 2S 0918-549,
  showing lines from a carbon-oxygen accretion disc. From
  \citet{njm+04}}
\label{fig:2003}
\end{figure}

The results of our 2003 programme are published in \citet{njm+04} and
are summarized in Fig.~\ref{fig:2003}. We identified the features in
the spectrum of 4U 0614+09 as relatively low ionization states of
carbon and oxygen. This clearly identifies this system as an UCXB and
suggests the donor in this system is a carbon-oxygen white dwarf. The
similarity of the spectrum of 4U 1543-624 suggests it is a similar
system, while for 2S 0918-549 the spectrum didn't have a high enough
S/N ratio to draw firm conclusions, but is also is consistent with
being a similar system (and clearly does not show the characteristic
strong hydrogen emission lines of low-mass X-ray binaries). We
therefore concluded that all these systems are UCXBs.


\subsection{The known UCXB 4U 1626-67 compared to 4U 0614-09}

\begin{figure}
  \includegraphics[angle=-90,width=0.7\textwidth]{nelemans_f2}
  \caption{VLT spectra of Comparison of the carbon-oxygen spectrum of
  4U 0614+09 with the spectrum of the 42 min system 4U 1626-67.}
\label{figL0614_1626}
\end{figure}

In Fig.~\ref{figL0614_1626} we compare the blue part of our 4U 0614+09
spectrum with the VLT spectrum of the 42 min. binary 4U 1626-67, which
harbours a 7 sec X-ray pulsar \citep{mmn81}. The similarities are
remarkable. This is interesting, as strong line emission in the X-ray
spectrum of 4U 1626-67 has been identified with O and Ne lines
\citep{sch+01}.


\subsection{The preliminary 2004 results}

The results of the 2004 programme will be published in a forthcoming
paper, but we will give some preliminary results here.

\subsubsection{4U 1822-00}

Due to the faintness of 4U 1822-00 its spectrum is of low
quality. However, just as with 2S 0918-549, the spectrum does not show
hydrogen or helium lines, making it clearly different from the spectra
of hydrogen rich systems. Provisionally we classify this system as an
UCXB.


\subsubsection{4U 1556-60}

\begin{figure}
  \includegraphics[angle=-90,width=0.8\textwidth]{nelemans_f3}
  \caption{Normalized spectrum of 4U 1556-60, showing strong H and
  HeII emission, plus the Bowen blend at 4640\AA.}
\label{1556}
\end{figure}

Based on its ``Ne feature'' 4U 1556-60 is a good UCXB
candidate. However, its optical spectrum (Fig.~\ref{1556}) shows a
classical low-mass X-ray binary spectrum with strong Balmer lines
(4101, 4340, 4961 and 6563 \AA) and lines from HeII (4686 (very
strong), 5411 and 6678 \AA). There is also strong emission at the
Bowen blend, a C and N complex around 4640\AA\ that is driven by He
fluorescence. This system thus probably is not an UCXB, suggesting that
the ``Ne feature'' is not a unique property of UCXBs.



\subsubsection{XB 1905+00}

%\begin{figure}
%  \includegraphics[height=.3\textheight]{1905_image}
%  \caption{Acquisition image for XB 1905+00, showing the source is
%  slightly extended, suggesting the counterpart of the X-ray source
%  might not be the G star. The scale of the image is XXX and north is
%  up, east to the left.}
%\label{fig:1905}
%\end{figure}%

The spectrum of XB 1905+00 shows the standard features of an early G
star. This puzzling result is possibly due to a chance alignment. The
acquisition image of the object obtained with a seeing of 0.6 arcsec
suggest the source actually is a blend of two objects. In that case
the optical counterpart of XB 1905+00 would be the fainter of the two
stars.  Note that this system was in quiescence at the time of our
observation, making the counterpart much fainter than when it was
found by \citet{ci85}.


\subsubsection{The known UCXB XB 1916-05}

\begin{figure}
  \includegraphics[angle=-90,width=0.7\textwidth]{nelemans_f4}
  \caption{Normalized (and rebinned) spectrum of the known UCXB XB
  1916-05, showing broad features, like 4U 0614, but possibly also
  hints of He and H emission.}
\label{fig:1916}
\end{figure}

In Fig.~\ref{fig:1916} we show the spectrum of the 50 min UCXB XB
1916-05. Again it shows some features that are very similar to the
spectra of 4U 0614+09 and 4U 1626-67, but in addition broad emission
around 4540\AA. A possible origin could be He, as there is a HeII line
at 4541\AA. Some of the other features also coincide roughly with
positions of HeII lines, but these are very close (within a few \AA)
of the Balmer lines, making identification very difficult. 


\section{Conclusions}

We have started a systematic study of the optical spectra of known and
candidate UCXBs. The first results are both interesting and promising:
we have confirmed the ultra-compact nature of many of the candidates
and have uncovered a variety of optical spectra, from pure
carbon-oxygen spectra to spectra probably still showing signs of
helium (and possibly hydrogen). One candidate (4U 1556-60) shows a
classical low-mass X-ray binary spectrum, making it unlikely that this
system is an UCXB. The faintness of these systems makes spectroscopic
period determination almost impossible, so that periods will have to
be found photometrically, as was recently done with 4U 1543-624 \citep{wc04}.



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nelemans.bbl
\begin{thebibliography}{13}
\expandafter\ifx\csname natexlab\endcsname\relax\def\natexlab#1{#1}\fi
\providecommand{\enquote}[1]{``#1''}
\expandafter\ifx\csname url\endcsname\relax
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\expandafter\ifx\csname urlprefix\endcsname\relax\def\urlprefix{URL }\fi
\providecommand{\eprint}[2][]{\url{#2}}

\bibitem[{Nelson} et~al.(1986)]{nrj86}
L.~A. {Nelson}, S.~A. {Rappaport}, and P.~C. {Joss}, \emph{\apj}, \textbf{304},
  231--240 (1986).

\bibitem[{Markwardt} et~al.(2002)]{mss+02}
C.~B. {Markwardt}, J.~H. {Swank}, T.~E. {Strohmayer}, J.~J.~M.~i. {Zand}, and
  F.~E. {Marshall}, \emph{\apjl}, \textbf{575}, L21--L24 (2002).

\bibitem[{Remillard} et~al.(2002)]{rss02}
R.~A. {Remillard}, J.~{Swank}, and T.~{Strohmayer}, \emph{\iaucirc},
  \textbf{7893}, 1 (2002).

\bibitem[{Markwardt} et~al.(2003)]{mjs03}
C.~B. {Markwardt}, M.~{Juda}, and J.~H. {Swank}, \emph{\iaucirc},
  \textbf{8095}, 2 (2003).

\bibitem[{Podsiadlowski} et~al.(2002)]{prp02}
P.~{Podsiadlowski}, S.~{Rappaport}, and E.~D. {Pfahl}, \emph{\apj},
  \textbf{565}, 1107--1133 (2002).

\bibitem[{Van Paradijs} and {McClintock}(1994)]{vm94}
J.~{van Paradijs}, and J.~E. {McClintock}, \emph{\aap}, \textbf{290}, 133--136
  (1994).

\bibitem[{Wang} and {Chakrabarty}(2004)]{wc04}
Z.~{Wang}, and D.~{Chakrabarty}, \emph{\apjl}, \textbf{616}, L139--L142 (2004).

\bibitem[Juett et~al.(2001)]{jpc00}
A.~M. Juett, D.~Psaltis, and D.~Chakrabarty, \emph{\apjl}, \textbf{560},
  L59--L63 (2001).

\bibitem[{Bildsten}(1995)]{bil95}
L.~{Bildsten}, \emph{\apj}, \textbf{438}, 852--875 (1995).

\bibitem[Nelemans et~al.(2004)]{njm+04}
G.~Nelemans, P.~G. Jonker, T.~R. Marsh, and M.~van~der Klis, \emph{\mnras},
  \textbf{348}, L7 (2004).

\bibitem[{Middleditch} et~al.(1981)]{mmn81}
J.~{Middleditch}, K.~O. {Mason}, J.~E. {Nelson}, and N.~E. {White},
  \emph{\apj}, \textbf{244}, 1001--1021 (1981).

\bibitem[Schulz et~al.(2001)]{sch+01}
N.~S. Schulz, D.~Chakrabarty, H.~L. Marshall, C.~R. Canizares, L.~C. Lee, and
  H.~J., \emph{\apj}, \textbf{563}, 941 (2001).

\bibitem[{Chevalier} and {Ilovaisky}(1985)]{ci85}
C.~{Chevalier}, and S.~A. {Ilovaisky}, \emph{Space Science Reviews},
  \textbf{40}, 443 (1985).

\end{thebibliography}
