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A 321.5 s modulation was discovered in 1999 in the X-ray light curve of HMCnc. In 2001 and 2002, optical photometric and spectroscopic observationsrevealed that HM Cnc is a very blue object with no intrinsic absorptions butbroad (FWHM 1500 km s^-1) low equivalent width emission lines (EW 1-6A), whichwere first identified with the HeII Pickering series. The combination of X-rayand optical observations pictures HM Cnc as a double degenerate binary hostingtwo white dwarfs, and possibly being the shortest orbital period binarydiscovered so far. The present work is aimed at studying the orbital motion ofthe two components by following the variations of the shape, centroid andintensity of the emission lines through the orbit. In February 2007, we carriedout the first phase resolved optical spectroscopic study with the VLT/FORS2 inthe High Time Resolution (HIT) mode, yielding five phase bins in the 321 smodulation. Despite the low SNR, the data show that the intensity of the threemost prominent emission lines, already detected in 2001, varies with the phase.These lines are detected at phases 0.2-0.6 where the optical emission peaks,and marginally detected or not detected at all elsewhere. Moreover, the FWHM ofthe emission lines in the phase resolved spectra is smaller, by almost a factor2, than that in the the phase-averaged 2001 spectrum. Our results areconsistent with both the pulsed optical component and emission linesoriginating in the same region which we identify with the irradiated surface ofthe secondary. Moreover, regardless of the origin of the un-pulsed opticalcontinuum, we note that the EWs of the emission lines might be up to -15 /-25A, larger than thought before; these values are more similar to thosedetected in cataclysmic variables. All the findings further confirm that the321s modulation observed in HM Cnc is the orbital period of the system, theshortest known to date.

VLT Phase Resolved Optical Spectroscopy of the Ultra-Compact Binary HM Cnc