[FLM99] Star A , the SIMBAD biblio

In April 1999 the transient low-mass X-ray binary X1658-298 resumed its strong and persistent X-ray emission after a 21 yr interval of quiescence. We present RXTE data obtained soon after the reappearance, including four eclipses with a mean duration of 901.9±0.8 s and ingress/egress times of 6-13 s. Our updated ephemeris for the source indicates that the 7.1 hr orbital period of the system is decreasing with a timescale of 10⁷ yr. Contemporaneous optical observations provide the first ever light curve of V2134 Oph, the optical counterpart of X1658-298. The optical modulation is highly variable from night to night and exhibits a distinct, narrow eclipse feature of about 0.2 mag superposed on a gradual brightness variation with ∼0.7-0.8 mag amplitude. Our data indicate that there is no significant offset between the time of mideclipse in the X-ray and optical and that the narrow optical eclipse feature is of the same duration as the X-ray eclipse. This implies an accretion disk structure characterized by enhanced optical emission coincident with the central X-ray-emitting area.