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The two peculiar Type Ic supernovae (SNe) 1997ef and 1997dq are shown to havevery similar photometric and spectral evolution in the epochs when both SNe areobserved (i.e. beyond $\sim 80$ days after the explosion). The early lightcurves and spectra of SN 1997ef suggested that this was a ``hypernova,'' or``SN 1998bw-like Type Ic supernova.'' The fact that the two SNe are verysimilar allows us to extend the time coverage of this type of event, since SN1997dq, unlike SN 1997ef, was observed well into the nebular phase. In contrastto SN 1998bw, the spectra of these two SNe did not become fully nebular untilalmost one year after the explosion. During a long transition phase, lasting atleast 6 months, the SNe developed nebular emission in lines of [\OI] and[\CaII], but at the same time they retained an underlying, photospheric-typespectrum, originating at very low velocities. Spectral synthesis techniques areused to model the spectrum of SN 1997dq, suggesting that it produced $\sim 0.16\Msun$ of \Nifs, and that a significant fraction of this is located in a dense,low-velocity inner region.Comment: 19 pages, 6 figures. To appear in the Astrophysical Journal (October  20, 2004

Properties of Two Hypernovae Entering the Nebular Phase: SN 1997ef and SN 1997dq