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Aim of this work is to determine C, N, and O abundances in stars of Galacticopen clusters of the Gaia-ESO survey and to compare the observed abundanceswith those predicted by current stellar and Galactic evolution models. In thispilot paper, we investigate the first three intermediate-age open clusters.High-resolution spectra, observed with the FLAMES-UVES spectrograph on the ESOVLT telescope, were analysed using a differential model atmosphere method.Abundances of carbon were derived using the C2 band heads at 5135 and 5635.5{\AA}. The wavelength interval 6470- 6490 {\AA}, with CN features, was analysedto determine nitrogen abundances. Oxygen abundances were determined from the [Oi] line at 6300 {\AA}. The mean values of the elemental abundances in Trumpler20 as determined from 42 stars are: [Fe/H] = 0.10 +- 0.08 (s.d.), [C/H] = -0.10+- 0.07, [N/H] = 0.50 +- 0.07, and consequently C/N = 0.98 +- 0.12. We measurefrom five giants in NGC 4815: [Fe/H] = -0.01 +- 0.04, [C/H] = -0.17 +- 0.08,[N/H] = 0.53 +- 0.07, [O/H] = 0.12 +- 0.09, and C/N = 0.79 +- 0.08. We obtainfrom 27 giants in NGC 6705: [Fe/H] = 0.0 +- 0.05, [C/H] = -0.08 +- 0.06, [N/H]= 0.61 +- 0.07, [O/H] = 0.13 +- 0.05, and C/N = 0.83 +- 0.19. The C/N ratios ofstars in the investigated open clusters were compared with the ratios predictedby stellar evolutionary models. For the corresponding stellar turn-off massesfrom 1.9 to 3.3 solar masses, the observed C/N ratio values are very close tothe predictions of standard first dredge-up models as well as to models ofthermohaline extra-mixing. The average [O/H] abundance ratios of NGC 4815 andNGC 6705 are compared with the predictions of two Galactic chemical evolutionmodels. The data are consistent with the evolution at the solar radius withinthe errors.

The Gaia-ESO Survey: CNO abundances in the open clusters Trumpler 20, NGC 4815, and NGC 6705