  Spectroscopy of the low mass X-ray binary Ser X-1 using the Gran Telescopio
Canarias have revealed a ~2 hr periodic variability that is present in the
three strongest emission lines. We tentatively interpret this variability as
due to orbital motion, making it the first indication of the orbital period of
Ser X-1. Together with the fact that the emission lines are remarkably narrow,
but still resolved, we show that a main sequence K-dwarf together with a
canonical 1.4Msun neutron star gives a good description of the system. In this
scenario the most likely place for the emission lines to arise is the accretion
disk, instead of a localized region in the binary (such as the irradiated
surface or the stream-impact point), and their narrowness is due instead to the
low inclination (<10 degrees) of Ser X-1.
