  The variability and rotation of ultra cool dwarfs (UCDs) provide important
information on their atmospheres and evolution. As part of an ongoing program
to investigate this, projected rotation speeds, vsini, are presented for 16
field UCDs (M9V-L7.5V). This doubles the number of L dwarfs for which vsini has
been measured. All targets are found to have vsini between 10 and 40 km/s,
confirming that L dwarfs are rapid rotators. From the random distribution of
the rotation axes, i, one-sided confidence intervals are placed on the rotation
periods of individual object. These are compared with published period data
obtained from photometric monitoring. From this, the period of 31 hrs for the
L0.5 dwarf 2M0746+2000 published by Gelino et al. (2002) may be ruled out as
the rotation period. The period of 11.2 +/- 0.8 hrs for the L1.5 dwarf
2M1145+2317 obtained by Bailer-Jones & Mundt (2001) is plausibly the rotation
period. The inclination of the rotation axis is i = 62-90 deg with an
expectation value of 76 deg. Alternatively the data set a lower limit on the
radius of 0.1 Rsol. Similarly, the period of 2.7 +/- 0.1 hrs for 2M1334+1940 is
the likely rotation period; the inclination is i = 27-44 deg (mean i = 34 deg).
Where no variability or period was detected by the monitoring programs the
likely reason is low contrast surface features. However, in three cases
variability but no period was detected, despite sensitivity to the likely
rotation periods inferred from vsini. This reinforces the `masking hypothesis',
the idea that the evolution of photospheric features obscures regular
modulation of the light curve. As has been previously discussed, a likely
candidate for such features is inhomogeneous dust clouds.
