The F2 supergiant HDE 258878 has recently been found to undergo deep e
clipses, with a periodicity Of 3 1/2 years. Its companion must be larg
e but of relatively low surface brightness. We have been able to measu
re the radial velocities of both components, which prove to be in a qu
ite eccentric orbit whose period and character agree with the eclipse
timings. The primary star is considerably more massive than its compan
ion, but both masses are large, 6 and 4 M. respectively. Photometric m
odelling of the eclipses shows that the radii of the components are 30
R. (primary) and 35 R. (secondary). Spectral lines of the secondary,
probably including the lithium line, have been detected in the red. Th
e stars are concluded to have types near to F2 Ib-II and G8 IIb, with
absolute magnitudes of -3m.6 and -1m.9 respectively. The distance of t
he system, estimated at 1.6 kpc, could be determined with certainty if
the angular separation of the components could be measured, as may be
come possible by optical interferometry quite soon. A series of occult
ations by the Moon will take place in 1995-97, but their circumstances
are generally unfavourable for critical studies with high time-resolu
tion. Both stars appear to be rotating faster than would correspond to
orbital pseudo-synchronization.