  Utilizing the Lorentz invariance of the correlation function we study the
effects of anisotropic transverse flow on the HBT correlation function. In
particular we show that directed flow would evidence itself by non zero
``side--long'' and ``side--out'' terms in the correlation function. We also
show that the study of ratios of the correlation functions evaluated on
different event subsamples and/or different Lorentz systems provides
information on the source dimensions and velocity which could be less biased by
corrections of different kinds. We present a fitting technique appropriate for
HBT analysis which does not require binning and is especially useful for
multidimensional fitting.
