  The CMa R1 star-forming region contains several compact clusters as well as
many young early-B stars. It is associated with a well-known bright rimmed
nebula, the nature of which is unclear (fossil HII region or supernova
remnant). To help elucidate the nature of the nebula, our goal was to
reconstruct the star-formation history of the CMa R1 region, including the
previously unknown older, fainter low-mass stellar population, using X-rays. We
analyzed images obtained with the ROSAT satellite, covering ~5 sq. deg.
Complementary VRI photometry was performed with the Gemini South telescope.
Colour-magnitude and colour-colour diagrams were used in conjunction with
pre-main sequence evolutionary tracks to derive the masses and ages of the
X-ray sources. The ROSAT images show two distinct clusters. One is associated
with the known optical clusters near Z CMa, to which ~40 members are added. The
other, which we name the "GU CMa" cluster, is new, and contains ~60 members.
The ROSAT sources are young stars with masses down to M_star ~0.5 M_sun, and
ages up to 10 Myr. The mass functions of the two clusters are similar, but the
GU CMa cluster is older than the cluster around Z CMa by at least a few Myr.
Also, the GU CMa cluster is away from any molecular cloud, implying that star
formation must have ceased; on the contrary (as already known), star formation
is very active in the Z CMa region.
