  The first micro-lensing event discovered towards the Small Magellanic Cloud
by the MACHO collaboration (Alcock et al. 1997b) had a very long time scale,
t_0 = 123 days. The EROS collaboration (Palanque-Delabrouille et al. 1997)
discovered a 2.5% brightness variation with a period P = 5.1 days. The OGLE
collaboration (Udalski et al. 1997) established that the variation persists
while the micro-lensing event is over, and the variable star is the one which
has been micro-lensed, not its blend.
  The simplest explanation of the periodic variability is in terms of a binary
star with the orbital period P(orb) = 10.2 days, with its component(s) tidally
distorted. Such objects are known as ellipsoidal variables. The binary nature
should be verified spectroscopically.
  Binary motion of the source introduces a parallactic effect into
micro-lensing light curve, and a few examples are shown. The effect is
relatively strong if the light center and the mass center of a binary are well
separated, i.e. if the binary has a large photometric dipole moment. The
diversity of binary parameters is large, and the corresponding diversity of
photometric effects is also large. The presence or absence of the effect may
constrain the lens mass and its distance from the source.

Astrophysics