Period--luminosity relation for persistent LMXBs in the near-infrared
  We study relations between the X-ray luminosity, orbital period and absolute
near-infrared magnitude of persistent low-mass X-ray binaries (LMXBs). We show
that often optical and near-infrared spectral energy distribution of LMXBs can
be adequately described by a simple model of an accretion disc and a secondary
star reprocessing X-ray emission of a central compact object. This gives us an
evidence that using an X-ray luminosity and an absolute infrared magnitude of a
persistent LMXB one can make reliable estimate of its orbital period. Using a
sample of well-known LMXBs, we have constructed a correlation of L_X, P_orb and
M_K values which can be approximated by a straight line with the RMS scatter at
the level of ~0.3 mag. Such a correlation, being to some extent an analogous to
the correlation, found by van Paradijs & McClintock 1994, might be helpful for
future population studies especially in the light of forthcoming surveys of the
Galaxy in X-ray and infrared spectral domains.
