  Extrasolar planets observation and characterization by high contrast imaging
instruments is set to be a very important subject in observational astronomy.
Dedicated instruments are being developed to achieve this goal with very high
efficiency. In particular, full spectroscopic characterization of low
temperature planetary companions is an extremely important milestone. We
present a new data analysis method for long slit spectroscopy (LSS) with
coronagraphy, which allows characterization of planetary companions of low
effective temperature. In a speckle-limited regime, this method allows an
accurate estimation and subtraction of the scattered starlight, to extract a
clean spectrum of the planetary companion. We performed intensive LSS
simulations with IDL/CAOS to obtain realistic spectra of low (R=35) and medium
(R=400) resolution in the J, H, and K bands. The simulated spectra were used to
test our method and estimate its performance in terms of contrast reduction and
extracted spectra quality. Our simulations are based on a software package
dedicated to the development of SPHERE, a second generation instrument for the
ESO-VLT. Our method allows a contrast reduction of 0.5 to 2.0 magnitudes
compared to the coronagraphic observations. For M0 and G0 stars located at 10
pc, we show that it would lead to the characterization of companions with Teff
of 600 K and 900 K respectively, at angular separations of 1.0 as. We also show
that errors in the wavelength calibration can produce significant errors in the
characterization, and must therefore be minimized as much as possible.
