= Solution
The true daily component has $f_D=1\,\mathrm{day^{-1}}$, above machine 3's <Nyquist frequency>. At sampling times $t_n=2n/3$ days,
$$
\cos(2\pi t_n)=\cos(4\pi n/3)=\cos(2\pi n/3).
$$
The last sequence is also produced by a continuous cosine with frequency $0.5\,\mathrm{day^{-1}}$ sampled at those same times. Hence the observed peak is the <sampled sinusoid alias>
$$
\boxed{f_{\rm alias}=|f_D-f_s|=0.5\,\mathrm{day^{-1}}.}
$$
It is not evidence for an additional two-day physical temperature cycle.
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