  X-ray surveys allow to identify young, main-sequence stars in the solar
neighborhood. Young, stellar samples, selected according to their activity, can
be used to determine the stellar birthrate in the last billion years. The ROSAT
North Ecliptic Pole survey (NEP), with its moderately deep sensitivity (fluxes
~10^(-14) erg cm^(-2) sec^(-1)), is the best survey, to date, able to sample
the intermediate-age (10^8 - 10^9 years) nearby population. The identification
process of NEP X-ray sources resulted in 144 X-ray sources having a normal
stellar counterpart, with an excess of yellow stars with respect to model
predictions. We want to determine if these X-ray active stars are young or
intermediate-age stars, or active binaries. We acquired high-resolution, high
signal-to-noise ratio optical spectra, to determine the age and physical
properties of the NEP X-ray-detected stellar sources. We measure the (i)
lithium abundance using the Li I 6707.8 angstrom line, which is an excellent,
youth indicator for our age range of interest; (ii) rotational and radial
velocities (through cross-correlation methods); and (iii) chromospheric
emission. The radial velocities distribution is consistent with that of a young
field star population of age 4x10^9 yrs, or younger. Rotational velocity
measurements imply that our sample is dominated by relatively young or
intermediate-age stars, as confirmed by our lithium measurements. Most of the
detected stars probably belong to a young or intermediate-age population. Our
measurements suggest that a burst in the stellar birthrate of a factor of four
occurred in the last 10^8 years. We cannot, however, exclude the possibility
that a small fraction of sources, amongst the fastest of the K-rotators, are
old binary systems with tidally-locked rotation.
