next in this series?

This is probably old hat for many, however....

What are the next three (or four or five or six) letters in this series?

o,t,t,f,f,s,s,e,n,t,e,t,

And -of course- why? 

(Results in 24hours.)

I have an idea, but I dont understand the 'o' and the last 'e'

t,f,f,s,s,e,n,t,t ...

I take it for granted that it has nothing to do with a language, like e.g. English, like "otb" "aso" "omg"...

That's English ?! 

The interest is so underwhelming that if I was caged I'd now be legs-up on the floor...

I still dont get the o and e

o???? twelve, thirteen.....nineteen, twenty e?????

one two three four five six seven eight nine ten eleven twelve. 

After a while I find it funny because of what happens to the series: 

t,f,f,s,s,e,n,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,t,f,f,f,f,f,f.....

rooperi... what can I say?  Although we both may be 'RodriguezOriginals' and possibly understand Cricket and Rugby and Colonial Cannon-Fodder but can I help it if you cant see the 'bush for the kokerbooms' ?! 

kapabl was the first correct: his modest response was a credit to him!
One, two, three...thirteen, fourteen, fifteen, etc... all the way and forever and ever.
A surprising number of clever folk miss it, even though its the most basic of all arithmetical series. A case of not seeing the wood for the trees ?
When I first saw it, I abandoned it after five minutes and then got it immediately the next morning: 'sleeping on it' so often really does work.

Timbeau in post 5 wonders what is English, like OTB  ---   and I wondered why O-T-T-F and so on might be abbreviations of English words, like done a lot, as there is OMG, OTB, ATM, IMO, BTW, OFC, LOL, DIY  ---  and more.  English is not my native tongue and I often wonder what something means, so BTW, lets try to keep all forums readable for everybody !

Sorry RomyGer, I thought I'd already responded to this...

Just the way it is, I guess mate. To have specified 'words' would have given it away even more easily. Why not do some in Dutch maybe? In fact you speak Dutch and English a million times better than I do just Dutch and would enjoy an even more unfair advatage. 

Any future series will -I hope- be language-neutral.