  Space-based transit surveys such as K2 and TESS allow the detection of small
transiting planets with orbital periods beyond 10 days. Few of these warm
Neptunes are currently known around stars bright enough to allow for detailed
follow-up observations dedicated to their atmospheric characterization. The
21-day period and 3.95 $R_\oplus$ planet HD106315c has been discovered based on
the observation of two of its transits by K2. We have observed HD106315 using
the 1.2m Euler telescope equipped with the EulerCam camera on two instances to
confirm the transit using broad band photometry and refine the planetary
period. Based on two observed transits of HD106315c, we detect its $\sim$1 mmag
transit and obtain a precise measurement of the planetary ephemerids, which are
critical for planning further follow-up observations. We have used the attained
precision together with the predicted yield from the TESS mission to evaluate
the potential for ground-based confirmation of Neptune-sized planets found by
TESS. We find that 1-meter-class telescopes on the ground equipped with precise
photometers could substantially contribute to the follow-up of 162 TESS
candidates orbiting stars with magnitudes of $V \leq 14$. Out of these, 74
planets orbit stars with $V \leq 12$ and 12 planets orbit $V \leq 10$, which
makes these candidates high-priority objects for atmospheric characterization
with high-end instrumentation.
