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"We've been invited to Rachel and Jamie's wedding - speaking of which, did you know that they're moving to Ealing?"

http://dictionary.cambridge.org/dictionary/british/speak_1

What does "speaking of which" mean in the sentence. Could you explain it please?
Comments  
All the words are used in their normal meanings. Which is a relative pronoun whose antecedent is the entire main clause (We've been invited to Rachel and Jamie's wedding). Another example:

I have been invited to his wedding, which is something I didn't expect. (= I have got an invitation but I didn't expect to get one.)

CB
Hello Cool Breeze,

Can you paraphrase the whole sentence please? I cannot find a relationship between the two sentences on either side of the "speaking of which".

We've been invited to Rachel and Jamie's wedding - speaking of which, did you know that they're moving to Ealing?
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Your English appears to be just about as good - or better - than mine, so you shouldn't have any insurmountable difficulties in understanding the sentence. English is slightly inexact occasionally, though. In this case, which can refer to the entire main clause as I suggested, or Rachel and Jamie's wedding. Whichever it refers to is inconsequential, though, as the idea of the wedding is what leads the speaker to say what he/she says after the phrase speaking of which.

There is only one sentence in your example. It consists of two clauses with the phrase speaking of which in between.

CB
You use the phrase "speaking of which" to introduce a related topic.

I've done the grocery shopping for the week, thank goodness. Oh, and speaking of which, while I was there, I ran into Mary in the parking lot, and she invited us to her and Tom's for dinner next Friday. I told her I'd let her know.

Speaking of which = speaking of my trip to the grocery store.

Don't forget you have to pick up Pat at band practice tonight at 8. Speaking of which, you'll probably need to fill up the car before you go -- I realized it was almost empty as I was pulling in the driveway.

Speaking which = speaking of taking the car to go get Pat.
"Speaking of which...." basically means, "while I was speaking about (Rachel and Jamie's wedding), I thought of something else I wanted to say about it."
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Thank you very much everybody