Context : Determining the dust properties and UV attenuation of distant star-forming galaxies is of great interest for our understanding of galaxy formation and cosmic star formation in the early Universe .
However , few direct measurements exist so far .

Aims : To shed new light on these questions we have targeted two recently discovered Lyman break galaxies ( LBGs ) at z \approx 6.8 and z = 7.508 to search for dust continuum and [ C ii ] \lambda 158 \mu m line emission .

Methods : The strongly lensed z \approx 6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703 , and the spectroscopically confirmed z = 7.508 LBG z8-GND-5296 in the GOODS-N field have been observed with the Plateau de Bure interferometer ( PdBI ) at 1.2mm .
These observations have been combined with those of three z > 6.5 Ly \alpha emitters ( named HCM6A , Himiko , and IOK-1 ) , for which deep measurements were recently obtained with the PdBI and ALMA .

Results : [ C ii ] is undetected in both galaxies , providing a deep upper limit of L _ { [ CII ] } < 2.8 \times 10 ^ { 7 } L _ { \odot } for A1703-zD1 , comparable to the non-detections of Himiko and IOK-1 with ALMA .
Dust continuum emission from A1703-zD1 and z8-GND-5296 is not detected with an rms of 0.12 and 0.16 mJy/beam .
From these non-detections and earlier multi-wavelength observations we derive upper limits on their IR luminosity and star formation rate , dust mass , and UV attenuation .
Thanks to strong gravitational lensing the achieved limit for A1703-zD1 is comparable to those achieved with ALMA , probing the sub-LIRG regime ( L _ { IR } < 8.1 \times 10 ^ { 10 } L _ { \odot } ) and very low dust masses ( M _ { d } < 1.6 \times 10 ^ { 7 } M _ { \odot } ) .
We find that all five galaxies are compatible with the Calzetti IRX– \beta relation , their UV attenuation is compatible with several indirect estimates from other methods ( the UV slope , extrapolation of the attenuation measured from the IR/UV ratio at lower redshift , and SED fits ) , and the dust-to-stellar mass ratio is not incompatible with that of galaxies from z = 0 to 3 .
For their stellar mass the high- z galaxies studied here have an attenuation below the one expected from the mean relation of low redshift ( z \la 1.5 ) galaxies .

Conclusions : More and deeper ( sub ) -mm data are clearly needed to directly determine the UV attenuation and dust content of the dominant population of high- z star-forming galaxies and to establish more firmly their dependence on stellar mass , redshift , and other properties .