the case of gamma-ray bursts (see @Stanek:2006gc), since no hard upper metallicity threshold prohibiting the production of a SN Ia progenitor system is known to exist. An exception to this may be found at very high metallicity, as evidenced by the abundance of He rather than CO WDs in the metal-rich cluster NGC 6791 (see @Kilic:2007yk) – very massive, metal-rich galaxies may show an additional deficit of SNe Ia beyond that of our simple model. This may even be evident in the data at the high-mass end of Fig. \[Iarate\], although it is difficult to draw a strong conclusion at present.

We also note that the model that we have used only results in a rather modest rate change with metallicity. It does not attempt to account for changes in the remnant mass that occur during the AGB phase of an isolated star (see, e.g., @Vassiliadis1993 [@Bird:2010yz; @Renedo:2010vb]), which could result in a larger metallicity effect. Since both SN Ia scenarios require binary evolution at some step in their evolution, this model should be adequate in this respect. We also have not attempted to vary the binary fraction with stellar mass or metallicity. This is not yet well understood either 