  SDSS J1021+1744 is a detached, eclipsing white dwarf / M dwarf binary
discovered in the Sloan Digital Sky Survey. Outside the primary eclipse, the
light curves of such systems are usually smooth and characterised by low-level
variations caused by tidal distortion and heating of the M star component.
Early data on SDSS J1021+1744 obtained in June 2012 was unusual in showing a
dip in flux of uncertain origin shortly after the white dwarf's eclipse. Here
we present high-time resolution, multi-wavelength observations of 35 more
eclipses over 1.3 years, showing that the dip has a lifetime extending over
many orbits. Moreover the "dip" is in fact a series of dips that vary in depth,
number and position, although they are always placed in the phase interval 1.06
to 1.26 after the white dwarf's eclipse, near the L5 point in this system.
Since SDSS J1021+1744 is a detached binary, it follows that the dips are caused
by the transit of the white dwarf by material around the Lagrangian L5 point. A
possible interpretation is that they are the signatures of prominences, a
phenomenon already known from H-alpha observations of rapidly rotating single
stars as well as binaries. What makes SDSS J1021+1744 peculiar is that the
material is dense enough to block continuum light. The dips appear to have
finally faded out around 2015 May after the first detection by Parsons et al.
in 2012, suggesting a lifetime of years.
