  We present medium-resolution VLT/FORS2 spectroscopy of six cataclysmic
variables discovered by the Sloan Digital Sky Survey. We determine orbital
periods for SDSS J023322.61+005059.5 (96.08 +/- 0.09 min), SDSS
J091127.36+084140.7 (295.74 +/- 0.22 min), SDSS J103533.02+055158.3 (82.10 +/-
0.09 min), and SDSS J121607.03+052013.9 (most likely 98.82 +/- 0.16 min, but
the one-day aliases at 92 min and 107 min are also possible) using radial
velocities measured from their H_alpha and H_beta emission lines. Three of the
four orbital periods measured here are close to the observed 75--80 min minimum
period for cataclysmic variables, indicating that the properties of the
population of these objects discovered by the SDSS are substantially different
to those of the cataclysmic variables found by other means. Additional
photometry of SDSS J023322.61+005059.5 reveals a periodicity of approximately
60 min which we interpret as the spin period of the white dwarf, suggesting
that this system is an intermediate polar with a low accretion rate. SDSS
J103533.02+055158.3 has a period right at the observed minimum value, a
spectrum dominated by the cool white dwarf primary star and exhibits deep
eclipses, so is an excellent candidate for an accurate determination of the
parameters of the system. The spectroscopic orbit of SDSS J121607.03+052013.9
has a velocity amplitude of only 13.8 +/- 1.6 km/s, implying that this system
has an extreme mass ratio. From several physical constraints we find that this
object must contain either a high-mass white dwarf or a brown-dwarf-mass
secondary component or both.
