Great news!
Reading around on other forums, I found out that we do not even need a receiver! We can just connect the "official" receiver to PC through the audio board to be able to detect the signals and reverse engineer it!
This is the result for an Oregon Scientific sensor (TH138) I have (for another weather station):
I used this sensor because it only transmits temperature values and because somebody else already examined a sensor similar to this one:
http://alyer.frihost.net/thn128decoding.htm
Examining the Auriol sensors will be a bit more tricky as they transmit quite complex data:
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I don't know which sensor(s) transmitted these data, as all of them were powered; it will be easy to grab just the udometer (by un-powering all the others), but it will be quite tricky to separate data for Temperature, Wind and umidity! :-?