Optical photometric monitoring of gamma-ray binaries
  Four gamma-ray binaries, namely PSR B1259-63, HESS J0632+057, HD 215227 and
LS I +61 303, contain compact objects orbiting around massive Be stars. The
nature of the compact object is only known in the case of PSR B1259-63, but the
other systems could also contain young non-accreting pulsars with relativistic
winds. Around periastron passage the compact objects should produce significant
changes in the structure of the Be discs due to gravitational forces and
eventually by ram pressure from the putative pulsar wind. Indeed, variability
in the Halpha emission line has been detected in all these systems, and
periodic variability in the optical photometry has been detected in two of
them. However, there is lack of a systematic monitoring with accurate
photometry, which could be used to constrain the shape of the disc during the
periastron passage. This information is important to build accurate physical
models to explain the broadband spectral energy distribution of these sources.
Here we present an ongoing program to monitor the optical photometry of
gamma-ray binaries and we show preliminary results for the case of HD 215227.
