XTE J1901+014 - the First Low-Mass Fast X-ray Transient?
  We continue to study the fast X-ray transient XTE J1901+014 discovered in 2002 by the RXTE observatory, whose nature has not yet been established. Based on the XMM-Newton observations of the source in 2006, we have obtained its energy spectrum, light curves, and power spectrum in the energy range 0.5-12 keV, which are in good agreement with our results obtained previously from the data of other observatories. In turn, this suggests that the source's emission is stable in the quiescent state. The XMM-Newton observations also allowed the source's localization accuracy to be improved to <2", which subsequently enabled us to search for its optical companion with the RTT-150 and 6-m BTA (Special Astrophysical Observatory) telescopes. Combining optical, X-ray, and infrared observations, we have concluded that the optical companion in the system under study can be either a later-type star at a distance of several kpc or a very distant red giant or an A or F star. Thus, XTE J1901+014 may be the first low-mass fast X-ray transient. 
D.I. Karasev, A. A. Lutovinov, R.A. Burenin (Space Research Institute,   Moscow, Russia)