THE REMARKABLE ECLIPSING ASYNCHRONOUS AM HERCULIS BINARY RX J19402-1025

We report on two years of photometric and spectroscopic observation of the recently discovered AM Herculis star RX J19402-1025. A sharp ecli pse feature is present in the optical and X-ray light curves, repeatin g with a period of 12116.290+/-0.003 s, The out-of-eclipse optical wav eform contains approximately equal contributions from a signal at the same period and another signal at 12150 s. As these signals drift in a nd out of phase, the wave form of the light curve changes in a complex but predictable manner. After one entire ''supercycle'' of 50 days (t he beat period between the shorter periods), the light curve returns t o its initial shape. We present long-term ephemerides for each of thes e periods. It is highly probable that the eclipse period is the underl ying orbital period, while the magnetic white dwarf rotates with P=121 50 s. The eclipses appear to be eclipses of the white dwarf by the sec ondary star. But there is probably also a small obscuring effect from cold gas surrounding the secondary, especially on the orbit-leading si de where the stream begins to fall towards the white dwarf, The latter hypothesis can account for several puzzling effects in this star, as well as the tendency among most AM Her stars for the sharp emission-li ne components to slightly precede the actual motion of the secondary. The presence of eclipses in an asynchronous AM Her star provides a mar velous opportunity to study how changes in the orientation of magnetic held lines affect the accretion flows. Repeated polarimetric light cu rves and high-resolution studies of the emission lines are now critica l to exploit this potential.