Observations of asteroids of type M have been one of my interests for a long time.
In a recent paper in
Astron. & Astrophysics, Suppl. Ser. 131, 55-62 the
from photometric lightcurve observations of nine M-type asteroids
are presented. New rotation
determined for 6 asteroids: 217 Eudora (12.54 h), 322 Phaeo (17.56 h),
572 Rebekka (5.65 h), 757 Portland (6.58 h), 857 Glasenappia (8.23 h) and
872 Holda (7.20 h). B-V colour measurements
of seventeen previously unclassified asteroids add seven asteroids to
the known M-type population.
An excess of fast rotators among
M-type asteroids compared to asteroids of other taxonomic types is evident.
The six asteroids
with slow spin rates, but hitherto classified as M, are shown to have
classification parameters untypical for the M-type population. In the three
figures below the normalized distributions of the spin rates are given
for asteroids belonging to taxonomic types M, S and C.