  Hydrogen gas evaporating from the atmosphere of the hot-Neptune GJ436b
absorbs over 50% of the stellar Ly$\alpha$ emission during transit. Given the
planet's atmospheric composition and energy-limited escape rate, this hydrogen
outflow is expected to entrain heavier atoms such as C and O. We searched for C
and Si in the escaping atmosphere of GJ436b using far-ultraviolet HST COS G130M
observations made during the planet's extended H I transit. These observations
show no transit absorption in the C II 1334,1335 \AA\ and Si III 1206 \AA\
lines integrated over [-100, 100] km s$^{-1}$, imposing 95% (2$\sigma$) upper
limits of 14% (C II) and 60% (Si III) depth on the transit of an opaque disk
and 22% (C II) and 49% (Si III) depth on an extended, highly asymmetric transit
similar to that of H I Ly$\alpha$. C$^+$ is likely present in the outflow
according to a simulation we carried out using a spherically-symmetric,
photochemical-hydrodynamical model. This simulation predicts a $\sim$2% transit
over the integrated bandpass, consistent with the data. At line center, we
predict the C II transit depth to be as high as 19%. Our model predicts a
neutral hydrogen escape rate of $1.6\times10^{9}$ g s$^{-1}$ ($3.1\times10^{9}$
g s$^{-1}$ for all species) for an upper atmosphere composed of hydrogen and
helium.
