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young transiting planets could play a key role to distinguish planet formation scenarios , as it is possible to test evolutionary models with the well determined parameters of such planets , like radius , mass , and age .
as no transiting planets with ages younger than @xmath0myr were found so far , we monitor young open clusters to search for transit signals among young stars . to increase phase coverage ,
continuous observations are needed .
therefore we established yeti ( young exoplanet transit initiative ) , a network of ground - based telescopes with mirror diameters of @xmath1 to @xmath2 m ( see neuhuser _ et al . _
@xcite for further details ) .
fig.[fig : yeti_map ] shows a map with all telescope sites as well as the telescope sizes .
each cluster is observed for at least three years with three campaign runs of lengths longer than one week .
observations are done in the @xmath3-band .
the first two target clusters of yeti were trumpler37 ( tr37 ) and 25ori .
some properties of both clusters are summarized in tab.[tab : cluster_prop ] .
the monitoring of tr37 started in summer 2009 at the university observatory jena .
data of the yeti telescopes were gathered in summers 2010 and 2011 , each year three campaign runs were performed .
the observations of 25ori at the university observatory jena started in january 2010 .
in this first phase of the observation two additional telescopes ( gunma / japan and cida / venezuela ) joined the photometric monitoring in january and february 2010 .
25ori became a target of the yeti project in the winter seasons 2010/2011 , 2011/2012 , and 2012/2013 .
fifteen different observatories , spread worldwide at different longitudes , participated in the campaigns .
data reduction ( dark / bias and flat - field correction ) as well as aperture photometry with adjusted aperture to the seeing conditions was done night by night and telescope by telescope . for a particular star ,
the final lightcurve was created by doing differential photometry ( broeg _ et al . _
@xcite ) for all nights and telescopes on a small sample of comparison stars ( similar brightness and color , small angular distance ) .
a detailed description can be found in errmann _
( @xcite ) .
lcc & trumpler37 & 25ori + age [ myr ] & @xmath4 [ 1 ] & 7 - 10 [ 2 ] + distance [ pc ] & @xmath5 [ 3 ] & @xmath6 [ 4 ] + number member stars & 774 [ 5 ] & @xmath7 [ 6 ] +   +    fig.[fig : timecoverage ] shows the time coverage of the observations for a star in tr37 for the third yeti campaign in 2011 . as the star is located in a larger separation to the cluster center , it does not fit into the field of view of some telescopes .
furthermore , not all telescopes could always allocate time and some suffer from seasonal bad weather , hence only 5 of the yeti sites are present .
but even with that smaller number of telescopes , we could observe nearly continuously for @xmath8h at the end of the campaign .
fig.[fig : phasecoverage ] shows the phase coverage of the same star . for a period up to 10d we reach 100% phase coverage and even for a period of 50d
the phase coverage is better than 70% .
the coverage at periods of a multiple of a full day is slightly worse , as a telescope in the pacific ocean is missing .
in the tr37 data two transit candidates were found using a simple box fitting algorithm .
another transiting candidate , first reported in van eyken _ et al . _
( @xcite ) , is also visible in the 25ori data from yeti .
at the late f - type star ( @xmath9mag ) , a transit - like signal of @xmath10mmag and period @xmath11d is visible . the time and phase coverage for this star is shown in fig.[fig : timecoverage ] and fig.[fig : phasecoverage ] , respectively .
the follow - up observations of this candidate are already finished and will be presented in detail in errmann _
( @xcite ) .
we got a high - quality light curve in the @xmath12-band and a low - resolution spectrum from _ cafos _ at the 2.2 m telescope at calar alto .
a medium - resolution spectrum was taken with _ hectochelle _ at mmt .
the host star has a late f spectral type .
eclipsing background stars in the optical point - spread function could be ruled out from high - resolution images from ircs observations at subaru . to solve the radial velocity orbit ,
5 high - resolution spectra were observed with _
hires _ at keck - i . as the latter observations were done near the quadratures , the line shifts are already visible by eye in fig .  [
fig : keck - spec ] .
the radial velocity variations of @xmath13km / s semi - amplitude are too large for a sub - stellar mass companion , hence we found a false positive .
the mass of the companion is about 0.15 to 0.35m@xmath14 .    adu . between the spectra of the two nights the shifts of the lines
are visible . ]
the @xmath15mag bright star shows a transit - like lightcurve with a period of @xmath16d and depth of @xmath17mmag .
additional , semi - periodic brightness variations of @xmath18mmag over time scale of @xmath19d are visible .
we are actively working on the follow - up observations of this candidate .
the host star ( cvso30 ) is a weak - lined t  tauri star and therefore very active .
this causes brightness variations up to @xmath20mmag , while the depth of the transit is only @xmath21mmag ( see fig.[fig : lc_25ori ] ) .
an interesting feature of the transit light curve of cvso30 was mentioned by van eyken _
( @xcite ) .
they observed two sets of light curves in the years 2009 and 2010 .
it can clearly be seen that there is an overall change in the transit shape between the two years data sets , which could be explained by gravity darkening of the host star ( barnes _ et al .
_ , @xcite ) .
further photometric follow - up can test the proposed spin - orbit misalignment .
the monitoring of the clusters trumpler37 and 25ori with yeti has already finished , while the monitoring of ic348 , collinder69 , ngc1980 , and ngc7243 is ongoing .
it is expected to find more transiting candidates , as we improve our routines and do further data analysis .
we thank all of the participating yeti observatories for the observations .
all the participating observatories appreciate the logistic and financial support of their institutions and in particular their technical workshops .
rn , mk , re , and sr would like to thank dfg for support in the priority programme spp 1385 on the _ first ten million years of the solar system _ in projects ne 515 / 33 - 1/2 and 34 - 1/2 .
we would like to acknowledge financial support from the thuringian government ( b 515 - 07010 ) for the stk ccd camera used in this project .
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to search for young transiting planets in continuous light curves , we monitor young open clusters ( 2 - 200myr ) with the yeti network . here 
 we report the first transiting candidates ( two in trumpler37 , one in 25ori ) . 
 follow - up observations of the candidates are partly done .