The gamma-ray source LSI+61 degrees 303 - I. RXTE/ASM observations

An analysis of RXTE/ASM observations of the unusual binary system LSI +61 d egrees 303 is presented. The X-ray light curve gives a binary period of 26. 42 +/- 0.05 days, and an eccentricity of 0.30 +/- 0.04 (1 sigma). The eccen tricity is determined by fitting an inverse Compton model to the X-ray ligh t curve. These are the best determinations to date, as the optical period a nd eccentricity have larger uncertainties and published values of the radio period are variable. The radio outburst peak is not phase locked to the bi nary period, so the radio period is not the same as the binary period.