The bug is the fact that it isn't sampling enough times to plot a decent graph. The period is is 0.7 and so is the Ts value. So therefore decrease the Ts value such that the function is sampled more times. It is possible to do this by dividing Ts by 100.
The New Code
F0 =13;
T0 =1/F0;
Ts = 0.07/100; % or equivalently Ts = 0.0007;
t = 0:Ts:13*T0;
x = real(exp(j*(2*pi*F0*t-pi/2)));
plot(t,x)
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