Context. 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. Aims. 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. Methods. 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. Results. Our method allows a contrast reduction of 0.5 to 2.0 mag 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 Te\! f\! f of 600 K and 900 K respectively, at angular separations of 1.0´´. 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
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