  The lack of hydrogen in spectra of type Ia supernovae (SN Ia) is often seen
as troublesome for single-degenerate (SD) progenitor models. We argue that,
since continued accretion of angular momentum can prevent explosion of the
white dwarf, it may be natural for the donor stars in SD progenitors of SN Ia
to exhaust their envelopes and shrink rapidly before the explosion. This
outcome seems most likely for SD SN Ia progenitors where mass-transfer begins
from a giant donor star, and might extend to other SD systems. Not only is the
amount of hydrogen left in such a system below the current detection limit, but
the donor star is typically orders of magnitude smaller than its Roche lobe by
the point when a SD SN Ia occurs, in which case attempts to observe collisions
between SN shocks and giant donor stars seem unlikely to succeed. We consider
the constraints on this model from the circumstellar structures seen in spectra
of SN 2006X and suggest a novel explanation for the origin of this material.
