  CAL 87 is one of the best known supersoft X-ray sources. However, the
measured masses, orbital period and orbital period evolution of CAL 87 cannot
be addressed by the standard thermal-timescale mass-transfer model for
supersoft X-ray sources. In this work we explore the orbital evolution of CAL
87 with both analytic and numerical methods. We demonstrate that the
characteristics mentioned above can be naturally accounted for by the
excited-wind-driven mass-transfer model.
