Context : Dust determines the temperature structure of protoplanetary disks , however , dust temperature determinations almost invariably rely on a complex modeling of the Spectral Energy Distribution .

Aims : We attempt a direct determination of the temperature of large grains emitting at mm wavelengths .

Methods : We observe the edge-on dust disk of the Flying Saucer , which appears in silhouette against the CO J=2-1 emission from a background molecular cloud in \rho Oph .
The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity allows us to directly measure the dust temperature .
The dust opacity can then be derived from the emitted continuum radiation .

Results : The dust disk absorbs the radiation from the CO clouds at several velocities .
We derive very low dust temperatures , 5 to 7 K at radii around 100 au , which is much lower than most model predictions .
The dust optical depth is > 0.2 at 230 GHz , and the scale height at 100 au is at least 8 au ( best fit 13 au ) .
However , the dust disk is very flat ( flaring index -0.35 ) , which is indicative of dust settling in the outer parts .

Conclusions :