  We present WHT and VLT spectroscopy of MM Ser, the optical counterpart to Ser
X-1. We deblend the red spectra of the two close stars identified by Wachter
(1997) and show that the brighter of the two is responsible for the Halpha and
HeI emission, hence confirming that this is the true counterpart of the X-ray
source. We also identify several HeII and NIII lines in the blue spectrum. The
isolated emission lines are all remarkably narrow, with FWHM 200-300 km/s. The
Bowen blend has structure suggesting that the individual components are also
narrow. These narrow lines could be from the disc if the binary inclination is
quite low, or they could come from a more localised region such as the heated
face of the companion star. Several interstellar lines are detected and
indicate that the reddening is moderate, and consistent with the neutral
hydrogen column density inferred in X-rays.
