  We present the results of our search for optical counterparts to high-mass
X-ray transient sources discovered by various X-ray missions. We obtained CCD
images of the X-ray fields through BVR and Halpha filters in order to identify
early-type stars in the R-Halpha versus B-V colour-colour diagram. We also
obtained medium-resolution spectroscopy of the candidates in order to confirm
the presence of H\alpha emission and perform spectral classification. We report
on the discovery of the optical counterparts to two X-ray sources: XTE
J1858+034 and IGR J01363+6610, and the follow-up observations of another two,
newly identified by our group: SAX J2103.5+4545 and GRO J2058+42. For another
source, IGR J00370+6122, we present the first detailed optical spectral
analysis. The optical photometry and spectroscopy reveal B-type companions in
all five sources; GRO J2058+42, SAX J2103.5+4545 and IGR J01363+6610 are
positively identified with Be/X-ray binaries, IGR J00370+6122 with a supergiant
X-ray binary, while the nature of XTE J1858+034 is uncertain. We also study the
relationship between the optical and X-ray emission during quiescent states.
