have lost most of their outer envelopes due to strong stellar winds or else from interaction with a close companion of about . Rapid mass loss can occur in the case of a Wolf–Rayet star, and these massive objects show a spectrum that is lacking in hydrogen. Type Ib progenitors have ejected most of the hydrogen in their outer atmospheres, while Type Ic progenitors have lost both the hydrogen and helium shells; in other words, Type Ic have lost more of their envelope (i.e., much of the helium layer) than the progenitors of Type Ib. In other respects, however, the underlying mechanism behind Type Ib and Ic supernovae is similar to that of a Type II supernova, thus placing Types Ib and Ic between Type Ia and Type II. Because of their similarity, Type Ib and Ic supernovae are sometimes collectively called Type Ibc supernovae. There is some evidence that a small fraction of the Type Ic supernovae may be the progenitors of gamma ray bursts (GRBs); in particular, type Ic supernovae that have broad spectral lines corresponding to high-velocity outflows are thought to be strongly associated with GRBs. However, it is also hypothesized that any hydrogen-stripped Type Ib or Ic supernova could be a GRB, dependent upon the geometry of the explosion. In any case, astronomers believe that most Type Ib, and probably Type Ic as well, result from core collapse in stripped, massive stars, rather than from the thermonuclear runaway of white dwarfs. As they are formed from rare, very massive stars, the rate of Type Ib and Ic supernova occurrence is much lower than the corresponding rate for Type II supernovae. They normally occur in regions of new star formation, and are extremely rare in elliptical galaxies. Because they share a similar operating mechanism, Type Ibc and the various Type II supernovae are collectively called core-collapse supernovae. In particular, Type Ibc may be referred to as stripped core-collapse supernovae. Light curves The light curves (a plot of luminosity versus time) of Type Ib supernovae vary in form, but in some cases can be nearly identical to those of Type Ia supernovae. However, Type Ib light curves may peak at lower luminosity and may be redder. In the infrared portion of the spectrum, the light curve of a Type Ib supernova is similar to a Type II-L light curve. Type Ib supernovae usually have slower decline rates for