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Paper text:
close - in exoplanets are expected to harbor extended atmospheres and in some cases lose mass through atmospheric evaporation , driven by x - ray and extreme uv emission from the host star .
direct observational evidence for such extended atmospheres has been collected at uv wavelengths .
however , the constraints on hydrogen densities in the extended atmosphere are weak , and depending on the ionization degree , can vary over several orders of magnitude @xcite .
we have conducted observational campaigns of transiting exoplanets in the soft x - ray regime ( 0.2 - 2 kev ) , where opacity is not caused by hydrogen , but by heavier elements such as oxygen , nitrogen , and carbon .
a growing number of transiting exoplanet hosts has been detected in x - rays now , for example gj  1214 @xcite , corot-7 , and gj  436 . for our present study we chose the very x - ray bright stars hd 189733 and corot-2 , both of which host a transiting hot jupiter . using the x - ray telescopes _ xmm - newton _ and _ chandra _ , we have repeatedly observed the transits of hd 189733b and corot-2b .
the x - ray images of the host stars are shown in fig .
[ images ] . in each single observation , the host star is clearly detected .
we then extracted x - ray light curves from each individual observation .
a typical example of a single light curve for each target is shown in fig .
[ single_lcs ] . in the case of hd 189733
, we observed small - scale stellar variability , but large flares are rare .
several small flares have been observed at other orbital phases , namely after the exoplanetary eclipse @xcite .
corot-2 has been previously observed in x - rays with a high x - ray luminosity of @xmath0 erg / s . in our observations
we confirm the high x - ray flux and additionally find several large flares ; an example is shown in fig .
[ single_lcs ] , right .
while the transit signal of the planet is not obvious in the individual light curves , we have detected the x - ray transit of hd 189733b in the stacked observations .
we show the resulting co - added light curve in fig .
[ transit_spin ] , left .
the x - ray transit is detected at a confidence of 99.8% .
the observed x - ray transit depth is 6 - 8% , depending on the assumed model @xcite .
we can theoretically expect a limb - brightened transit model with a `` w''-like shape @xcite , because the corona is optically thin and therefore limb - brightened ( in contrast to the limb - darkened photosphere ) .
however , the signal - to - noise is not high enough to reliably discriminate between limb - brightened and limb - darkened models . from the detected transit depth
, we infer a minimum extent of the exoplanetary atmnosphere out to ca .  1.75 optical planetary radii , with particle number densities of the order of ca .
@xmath1 @xmath2 when assuming an atmospheric metallicity of ten times solar .         for corot-2b ,
the individual light curves are much more dominated by intrinsic stellar variability , as the host star is more x - ray luminous by a factor of ten comapred to hd 189733 .
detailed data analysis is still ongoing in order to identify suitable energy bands to minimize the influence of impulsive flares on the light curves .
for x - ray studies of exoplanets it is helpful to have a relatively x - ray bright host star with low intrinsic variability .
magnetic activity is the main source of both variability and x - ray emission for cool stars , and it is an intriguing question if close - in exoplanets can actually have an effect on the stellar activity though tidal or magnetic star - planet interactions @xcite .
observational indications for such an ( intermittent ) magnetic effect on the chromospheres of exoplanet host stars have been reported early on for two systems @xcite .
indications for tidal effects have been found through observations of stellar rotation periods and velocities @xcite as well as orbital obliquities of exoplanets @xcite .    to exclude observational biases from distorting the sample of host stars and their activities , which can arise due to the fact that small exoplanets often go unnoticed around active stars @xcite
, we have developed a new approach to test for a planet - induced high stellar activity level .
we have selected wide ( several 100 au ) stellar binaries in which only one of the stellar components is known to host an exoplanet .
we have then obtained x - ray observations of the two stars in each binary system . with a known spectral type , age - activity relationships ( * ? ? ?
* ; * ? ? ?
* ; * ? ? ?
* for example ) allow to test if the two stars display activity levels which are consistent with a common age .
we have used the x - ray luminosity of the secondary , which is the star without a known planet in our sample of five wide binaries , to estimate the age of the respective wide binary system .
we then compare the observed x - ray luminosity of the planet - hosting star to the x - ray luminosity expected for that age .
we find that for the two systems in which strong tidal interaction between exoplanet and host star is expected , the planet - hosting stars are over - luminous in x - rays by 1 - 2 orders of magnitude ( see fig .
[ transit_spin ] , right ) . in three other systems in which the tidal interaction between star and planet is expected to be weak ,
no such discrepancy is observed .
this points towards a possible spin - up , or inhibited spin - down , of stars which host tidally strongly interacting hot jupiters .
other possible explanations , such as high - amplitude stellar activity cycles , are unlikely due to the observed constancy of the stellar x - ray emission level over several years
. observations of a larger sample of wide planet - hosting binaries are in progress .
planets in close orbits around their host stars are subject to strong irradiation . 
 high - energy irradiation , originating from the stellar corona and chromosphere , is mainly responsible for the evaporation of exoplanetary atmospheres . 
 we have conducted multiple x - ray observations of transiting exoplanets in short orbits to determine the extent and heating of their outer planetary atmospheres . in the case of hd 189733 b , 
 we find a surprisingly deep transit profile in x - rays , indicating an atmosphere extending out to 1.75 optical planetary radii . 
 the x - ray opacity of those high - altitude layers points towards large densities or high metallicity . 
 we preliminarily report on observations of the hot jupiter corot-2 b from our large program with xmm - newton , which was conducted recently . 
 in addition , we present results on how exoplanets may alter the evolution of stellar activity through tidal interaction .