ॐ स्त्रा स्त्रीं स्त्रौं सः केतुवे नमः  
This time I have got the spelling correct for seh  सः  and nameh नमः
BUT while I have got the straa स्त्रा of straan right, there is no dot above the aa.
I hope that is helpful all the same

Praying to Shri Ketu

Om Straam Streem Stroum Seh Kétuvé Nameh is the correct prayer to Shri Kétu ji.

               Several English language websites and blogs have, obviously copying each other, incorrectly recorded the prayer as ‘Om Shraam Shreem Shroum Seh Kétuvé Nameh’ or even  Kétavé. The latter mantr is basically the prayer to the greh/ grih (‘planet’) Shri Chañdr ji, which is

            ‘Om Shraam Shreem Shroum Seh Chañdr-masé Nameh.’

               I would humbly request the creators of all these sites and blogs to please make this correction.

               The ‘m’ at the end of Straam, Streem and Stroum is nasalized, like the ‘n' in ‘song.’ It is not a full ‘m.’ The ‘t’ is soft as in tapasvi.

               I have cross-checked with someone who comes from a long line of religious scholars and have confirmed that the correct Kétu Beej Mantr is, indeed, Om Straam Streem Stroum Seh Kétuvé Nameh.

               May Shri Kétu ji be pleased by your prayer.

 ॐ स्त्रआं स्त्रीं स्त्रौं सह केतुवे नमह 

is the nearest I could get with the Blogspot Hindi service. In स्त्रआं the aa has been capitalised in Devanagari.

But you get the correct pronunciation even with this spelling.