The transient X-ray source SAX J2239.3+6116 and its optical counterpart

We have discovered with the Wide Field Cameras on board BeppoSAX the weak t ransient X-ray source SAX J2239.3+6116 whose position coincides with that o f 4U 2238+60/3A 2237+608 and is close to that of the fast transient AT 2238 +60 and the unidentified EGRET source 3EG 2227+6122. The data suggest that the source, exhibits outbursts that last for a few weeks and peak to a flux of 4 x 10(-10) erg cm(-2) s(-1) (2-10 keV) at maximum. During the peak the X-ray spectrum is hard with a photon index of -1.1 +/- 0.1. Follow-up obse rvations with the Narrow-Field Instruments on the same platform revealed a quiescent emission level that is 10(3) times less. Searches through the dat a archive of the All-Sky Monitor on RXTE result in the recognition of five outbursts in total from this source during 1996-1999, with a regular interv al time of 262 days. Optical observations with the KPNO 2.1 m telescope pro vide a likely optical counterpart. It is a B0 V to B2 III star with broaden ed emission lines at an approximate distance of 4.4 kpc. The distance impli es a 2-10 keV luminosity in the range from 10(33) to 10(36) erg s(-1). The evidence suggests that SAX J2239.3+6116 is a Be X-ray binary with an orbita l period of 262 days.