  We present the first ever discovery of a short-period and unusually
helium-deficient dwarf nova KSP-OT-201701a by the Korea Microlensing Telescope
Network Supernova Program. The source shows three superoutbursts, each led by a
precursor outburst, and several normal outbursts in BVI during the span of ~2.6
years with supercycle and normal cycle lengths of about 360 and 76 days,
respectively. Spectroscopic observations near the end of a superoutburst reveal
the presence of strong double-peaked HI emission lines together with weak HeI
emission lines. The helium-to-hydrogen intensity ratios measured by
HeI{\lambda}5876 and H{\alpha} lines are 0.10 {\pm} 0.01 at a quiescent phase
and 0.26 {\pm} 0.04 at an outburst phase, similar to the ratios found in
long-period dwarf novae while significantly lower than those in helium
cataclysmic variables (He CVs). Its orbital period of 51.91 {\pm} 2.50 minutes,
which is estimated based on time series spectroscopy, is a bit shorter than the
superhump period of 56.52 {\pm} 0.19 minutes, as expected from the
gravitational interaction between the eccentric disk and the secondary star. We
measure its mass ratio to be 0.37^{+0.32}_{-0.21} using the superhump period
excess of 0.089 {\pm} 0.053. The short orbital period, which is under the
period minimum, the unusual helium deficiency, and the large mass ratio suggest
that KSP-OT-201701a is a transition object evolving to a He CV from a
long-period dwarf nova with an evolved secondary star.
