filtering out harmonic trend

[Francesco Potorti`]

I have a 200,000 samples measurement, which is a sort of noise with
bandwidth in the order of the inverse of 500 samples.  Superimposed to
this noise there is a very slow sinusoid with a period of about 300,000
samples.  This means that my measurements do not even see a complete
cycle of this sinusoid.

I want to estimate the sinusoid (in order to remove it from the
measurement).  So I need some sort of very-low pass filter.  However,
usign a simple causal low-pass filter would give me a delayed output.
Can anyone suggest some keyword for a noncausal filter to look for?  I
guess that it could be simple, because the bandwidths of signal and
noise are so far away each other.

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