We present medium-resolution VLT/FORS2 spectroscopy of six cataclysmic variables (CVs) discovered by the Sloan Digital Sky Survey (SDSS). We determine orbital periods for SDSSJ023322.61+005059.5 (96.08 +/- 0.09min), SDSSJ091127.36+084140.7 (295.74 +/- 0.22min), SDSSJ103533.02+055158.3 (82.10 +/- 0.09min) and SDSSJ121607.03+052013.9 (most likely 98.82 +/- 0.16min, but the one-day aliases at 92 and 107min are also possible) using radial velocities measured from their Hα and Hβ emission lines. Three of the four orbital periods measured here are close to the observed 75-80min minimum period for CVs, indicating that the properties of the population of these objects discovered by the SDSS are substantially different to those of the CVs found by other means. Additional photometry of SDSSJ023322.61+005059.5 reveals a periodicity of approximately 60min which we interpret as the spin period of the white dwarf, suggesting that this system is an intermediate polar with a low accretion rate. SDSSJ103533.02+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 SDSSJ121607.03+052013.9 has a velocity amplitude of only 13.8 +/- 1.6kms-1, 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.