  We present optical (BVRI) photometric measurements of a sample of 76 common
proper motion wide separation main sequence binary pairs. The pairs are
composed of a F-, G-, or K-type primary star and an M-type secondary. The
sample is selected from the revised NLTT catalog and the LSPM catalog. The
photometry is generally precise to 0.03 mag in all bands. We separate our
sample into two groups, dwarf candidates and subdwarf candidates, using the
reduced proper motion (RPM) diagram constructed with our improved photometry.
The M subdwarf candidates in general have larger $V-R$ colors than the M dwarf
candidates at a given $V-I$ color. This is consistent with an average
metallicity difference between the two groups, as predicted by the
PHOENIX/BT-Settl models. The improved photometry will be used as input into a
technique to determine the metallicities of the M-type stars.
