Life after eruption - III. Orbital periods of the old novae V365 Car, AR Cir, V972 Oph, HS Pup, V909 Sgr, V373 Sct and CN Vel

We present time-series photometric and spectroscopic data for seven old novae. They are used to derive the orbital period for the systems: V365 Car (5.35 h), AR Cir (5.14 h), V972 Oph (6.75 h), HS Pup (6.41 h), V373 Sct (3.69 h), V909 Sgr (3.43 h) and CN Vel (5.29 h). Their addition increases the number of orbital periods for novae by ∼10 per cent. The eclipsing nature of V909 Sgr is confirmed, and in three other cases (V365 Car, Ar Cir and V373 Sct) we detect significant photometric orbital variability with amplitudes ≥0.2 mag in R. The resulting period distribution is briefly discussed. We furthermore provide new measurements for the previously ambiguous coordinates for AR Cir and CN Vel and the identification of a new probable W UMa variable in the field of V909 Sgr. The spectrum of V972 Oph presents an emission feature redward of Hα which we tentatively identify with the C II λλ6578/6583 doublet. It is shown that this line originates in the binary and not in a shell, and to our knowledge this is the first time that it has been detected in such quality in a cataclysmic variable (CV). We argue that this line could be more common in CVs, but that it can be easily masked by the broad Hα emission that is typical for these systems. A closer inspection of the line profiles of the other novae indeed reveals an extended red wing in V365 Car, CN Vel and AR Cir. In the latter system, additionally an absorption counterpart blueward of Hα is detected and thus in this case a bipolar outflow appears as a more likely scenario rather than C II emission.