We present spectroscopic and photometric observations of the luminous narrow-line Type IIP ( plateau ) supernova 1994W .
After the plateau phase ( t \gtrsim 120 days ) , the light curve dropped by \sim 3.5 mag in V in only 12 days .
Between 125 and 197 days after explosion the supernova faded substantially faster than the decay rate of ^ { 56 } Co , and by day 197 it was 3.6 magnitudes less luminous in R compared to SN 1987A .
The low R -luminosity could indicate \lesssim 0.0026 ^ { +0.0017 } _ { -0.0011 } ~ { } M _ { \odot } of ^ { 56 } Ni ejected at the explosion , but the emission between 125 and 197 days must then have been dominated by an additional power source , presumably circumstellar interaction .
Alternatively , the late light curve was dominated by ^ { 56 } Co decay .
In this case , the mass of the ejected ^ { 56 } Ni was 0.015 ^ { +0.012 } _ { -0.008 } ~ { } M _ { \odot } , and the rapid fading between 125 and 197 days was most likely due to dust formation .
Though this value of the mass is higher than in the case with the additional power source , it is still lower than estimated for any previous Type II supernova .

Only progenitors with M _ { ZAMS } \sim 8 - 10 ~ { } M _ { \odot } and M _ { ZAMS } \gtrsim 25 ~ { } M _ { \odot } are expected to eject such low masses of ^ { 56 } Ni .
If M _ { ZAMS } \sim 8 - 10 ~ { } M _ { \odot } , the plateau phase indicates a low explosion energy , while for a progenitor with M _ { ZAMS } \gtrsim 25 ~ { } M _ { \odot } the energy can be the canonical \sim 10 ^ { 51 } ergs .
As SN 1994W was unusually luminous , the low-mass explosion may require an uncomfortably high efficiency in converting explosion energy into radiation .
This favors a M _ { ZAMS } \gtrsim 25 ~ { } M _ { \odot } progenitor .

The supernova ’ s narrow ( \sim 1000 ~ { } km~ { } s ^ { -1 } ) emission lines were excited by the hot supernova spectrum , rather than a circumstellar shock .
The thin shell from which the lines originated was most likely accelerated by the radiation from the supernova .