  We analyze two multi-chord stellar occultations by Pluto observed on July
18th, 2012 and May 4th, 2013, and monitored respectively from five and six
sites. They provide a total of fifteen light-curves, twelve of them being used
for a simultaneous fit that uses a unique temperature profile, assuming a clear
(no-haze) and pure N_2 atmosphere, but allowing for a possible pressure
variation between the two dates. We find a solution that fits satisfactorily
(i.e. within the noise level) all the twelve light-curves, providing
atmospheric constraints between ~1,190 km (pressure ~ 11 \mubar) and ~ 1,450 km
(pressure ~0.1 \mubar) from Pluto's center. Our main results are: (1) the
best-fitting temperature profile shows a stratosphere with strong positive
gradient between 1,190 km (at 36 K, 11 \mubar) and r = 1,215 km (6.0 \mubar),
where a temperature maximum of 110 K is reached; above it is a mesosphere with
negative thermal gradient of -0.2 K/km up to ~ 1,390 km (0.25 \mubar), where,
the mesosphere connects itself to a more isothermal upper branch around 81 K;
(2) the pressure shows a small (6 %) but significant increase (6-\sigma level)
between the two dates; (3) without troposphere, Pluto's radius is found to be
R_P = 1,190 +/- 5km. Allowing for a troposphere, R_P is constrained to lie
between 1,168 and 1,195 km; (4) the currently measured CO abundance is too
small to explain the mesospheric negative thermal gradient. Cooling by HCN is
possible, but only if this species is largely saturated; Alternative
explanations like zonal winds or vertical compositional variations of the
atmosphere are unable to explain the observed mesospheric trend.
