corresponding to $A_{V}$=0.40 ($E_{B-V}$=0.13 for R=3.1). For blue stars $E_{B-V}$=0.13 equates to $E_{g-r}$=0.13. If we add a conservative uncertainty of 0.13 mag in our observed colours (Barclay et al 2011) our search region therefore covers $-0.12<(g-r)<0.40$, while the brightness of stars were in the range $15<g<23$.

Here we note that the shape of the light curves and the colour of our sources is similar to some cataclysmic variables (CVs; eg Szkody et al 2002 who present the first sample of CVs discovered using SDSS data). The hydrogen accreting CVs have a minimum orbital period of $\sim$80 mins (Gänsicke et al 2009), implying a possible over-lap with our target selection. (The helium dominated CVs with orbital periods in the range 40–70 mins do not show a photometric modulation implying we are not overlapping with these objects). However, CVs show optical spectra dominated by line emission – although we obtained a spectrum for only 7 out of the 31 new sources – none show evidence for emission lines (cf §\[spectra\]). Moreover, although pulsating blue stars are not X-ray sources, CVs are weak to moderately strong X-ray sources (Verbunt et al 1997). We therefore cross-correlated the position of our sources with that of 