Is it possible to climb Pico de Aneto (the highest mountain in Pyrenees) without any special equipment in September (i.e. without crampons etc.) or is the terrain too snowy? Is it possible to do it in one day, considering one will take the bus to the nearest possible bus stop to the trail up the mountain?
Searching for "Pico de Aneto" "without crampons" gives this UK Climbing result:
Aneto - much the same as what's already said. Plenty of people were doing it in the middle of July last year without axe or crampons - but that would be a bit hairy for my taste. The last snow to the summit is steep, and if you did slip, stopping without an axe would very difficult, and you'd end up a long way down the glaicer.
That link also mentions a magazine and book on the area that might be useful.
Most sites say that crossing the glacier and the final summit with crampons and/or ice axe is recommended.
I'm sure some people get away with it:
After the initial glacier section, the glacier steepens a little and has a covering of snow, then a rocky rib is crossed and the final steep(ish) section is climbed. We were surprised to see a couple of parties descending (very cautiously!) without crampons. To take crampons and use the shorter route would be our recommendation.
There are various routes you could take and sites give an total ascent and descent times in the region of 10-14 hours.
I suggest it's probably good to do a little more research before you set off though!
Disclaimer: I haven't done it.