SPECTRAL SIGNATURES OF THE FAST OPTICAL OSCILLATIONS IN THE AM HERCULIS SYSTEM AN URASE MAJORIS

We report on simultaneous photometric and spectroscopic observations o f AN UMa obtained in high time resolution mode at the Russian 6m teles cope on March 1991 and January 1992. These observations are the first detailed spectroscopic data obtained since the discovery period of the source. A study of the Balmer and helium emission line velocities rev eals a significant phase shift with time, attributed to an inaccuracy in the rotational/orbital period. A new ephemeris is derived with a re fined period of (6890.6436 +/- 0.0035)s. With this new period all publ ished photometric light curves are correctly phased over more than 15 years and, in particular, the phase discrepancy previously reported in X-ray light curves is eliminated. The geometry of the system appears stable. A modelling of the narrow and broad emission line velocities a llows to constrain the system parameters. The best solution is found f or a low mass white dwarf (M(wd) = 0.4-0.6M.), an inclination i = 40 - 60 degrees, an angle of the magnetic field axis with respect to the p erpendicular to the orbital plane of theta(d) = 25 - 45 degrees and th e narrow component of the emission lines produced in a stream near the capture radius. Quasi-periodic oscillations are detected in the Johns on-B light curve during all parts of the observations with rms amplitu des varying from 1.6 to 3.9 %. They are comparable to what already rep orted for this source and appear as a broad excess of 0.18 to 0.48 Hz (FWHM) around mean frequencies from 0.62 to 0.72 Hz. Superposed to the se low amplitude background oscillations, short duration events are al so found. They consist of short-lived (30s) pulse shot of high amplitu de oscillations confined in a very narrow range of frequencies. We rep orted here on the first spectral characteristics of these oscillations in the (4100-5100 Angstrom) range. Contrary to what expected by therm al instability models, there seems to be no counterpart of these pecul iar narrow frequency oscillations in the continuum bands but a signifi cant power is present in some of the emission lines bands. We suggest that these peculiar oscillations may be of different types, produced c lose to the capture radius, in the turbulent region where the accretio n stream interacts with the magnetic field.