It came from a supermarket, it was very small and not taller than about 15cm (1/2 ft) when i got it, maybe it would have grown taller if I hadn't over watered it in a too big pot. The heart of the flower seems unusual large and the petals short for a Cosmos..

Might be Rubenza? but I am not sure.

The colour certainly matches Cosmos Rubenza, Caroline, it is just that the petals on mine are so short and shaped a little like the 'Sea Shell' series and the heart so big in relation..I bought it in bud and didn't know how it would turn out, in fact I confess I was disappointed..

Cosmos bipinnatus Sonata Series is dwarf, up to 30cm ..

https://www.rhs.org.uk/Plants/...

http://www.chilternseeds.co.uk...

That's the only one which fits out of the many listed on Chiltern Seeds, but it's no guarantee. The petals on yours do appear to be shorter and slightly fluted, with a larger centre as you noted Myriam.

http://www.chilternseeds.co.uk...

This site links to several sites with a list of the most used bedding plants at the bottom, I don't know what else it can be.

http://www.beddingplantsuk.co....

Cosmos bipinnatus, 'Sonata' Series, 'Sonata Carmine' is the best fit Janet, apart from the too large heart and the shorter petals, the photo of it in the chilternseeds site also show the slightly fluted petals..
It could be the growing condition or that it had been forced to bloom by the nursery that made mine look a bit odd.., if it had lived longer it might have shown its true shape..

It probably suffered as you say Myriam, from dwarf dwarfism. Look at the photo on pinterest, one flower in particular towards the back left/top you can see a flower with shorter petals and a larger centre. There must be some variation in seed grown plants.

https://www.google.co.uk/searc...

Another thing which I thought of is the sprays used to make plants more dwarf!

http://msue.anr.msu.edu/news/o...

I suppose you meant this one,
https://www.google.co.uk/searc...
the photo showing where it says: Cosmos bipinnatus hybrid with
Cosmos atrosanguineus. Yes, that really fits mine in all aspects!

I really don't like that nurseries use that stuff to inhibit natural growth in plants, I always hope the plant quickly grows out of that growth inhibitor to look more natural..

Thanks a lot! I consider this one as solved!

edit because the info of it being a hybrid is wrong, it is a compact series of Cosmos bipinnatus..