Aligning different for each pair of equations in same align-environment

Question

I am using the following environment and want to have alignment for each the first two, the second two and the third two equations at the =-Symbol. In this moment everything aligns at the same point but this way the last equation for instance is too far on the left. What would you suggest? Splitting it up in three align-environments doesn't help as then the seperation between the equations would be too big.

\begin{align}
    Cov\left(\tilde{c}_D,\tilde{r}_M\right) &= \sum_{i=1}^4f_i\left(c_{D,i}-E(\tilde{c}_D)\right)\left(r_{M,i}-E(\tilde{r}_M)\right),\\
    Cov\left(\tilde{c}_E,\tilde{r}_M\right) &= \sum_{i=1}^4f_i\left(c_{E,i}-E(\tilde{c}_E)\right)\left(r_{M,i}-E(\tilde{r}_M)\right),\\
    V_D &= \frac{E(\tilde{c}_D) - \left( E\left(\tilde{r}_M\right) - r_f\right)\frac{Cov\left(\tilde{c}_D,\tilde{r}_M\right)}{\sigma_{\tilde{r}_M}^2}}{1+r_f},\\
    V_U &= \frac{E(\tilde{c}_E) - \left( E\left(\tilde{r}_M\right) - r_f\right)\frac{Cov\left(\tilde{c}_E,\tilde{r}_M\right)}{\sigma_{\tilde{r}_M}^2}}{1+r_f},\\
    \mu_D &= \frac{E(\tilde{c}_D)}{V_D} - 1 \text{~und~}\\
    \mu_E &= \frac{E(\tilde{c}_E)}{V_E} - 1.
\end{align}

Answer 1

ok, I found a solution by myself as I googled for another problem. This helps:

\begin{gather}
    \begin{align}
        Cov\left(\tilde{c}_D,\tilde{r}_M\right) &= \sum_{i=1}^4f_i\left(c_{D,i}-E(\tilde{c}_D)\right)\left(r_{M,i}-E(\tilde{r}_M)\right),\\
        Cov\left(\tilde{c}_E,\tilde{r}_M\right) &= \sum_{i=1}^4f_i\left(c_{E,i}-E(\tilde{c}_E)\right)\left(r_{M,i}-E(\tilde{r}_M)\right),
    \end{align}\\
    \begin{align}
        V_D &= \frac{E(\tilde{c}_D) - \left( E\left(\tilde{r}_M\right) - r_f\right)\frac{Cov\left(\tilde{c}_D,\tilde{r}_M\right)}{\sigma_{\tilde{r}_M}^2}}{1+r_f},\\
        V_U &= \frac{E(\tilde{c}_E) - \left( E\left(\tilde{r}_M\right) - r_f\right)\frac{Cov\left(\tilde{c}_E,\tilde{r}_M\right)}{\sigma_{\tilde{r}_M}^2}}{1+r_f},
    \end{align}\\
    \begin{align}
        \mu_D &= \frac{E(\tilde{c}_D)}{V_D} - 1 \text{~und~}\\
        \mu_E &= \frac{E(\tilde{c}_E)}{V_E} - 1.
    \end{align}
\end{gather}

Answer 2

I suggest some improvements: using subequations since the equations go in pairs,medium-sized fractions from nccmath for fractions in fractions, declare Cov as a math operator (for now, Cov looks like the product of three variables), and another placement of the word ‘und’ with the ArrowBetweenLines command from mathtools:

\documentclass{article}
\usepackage{mathtools, nccmath}
\DeclareMathOperator{\Cov}{Cov}

\begin{document}

\begin{gather}
  \begin{subequations}
    \begin{align}
      \Cov\left(\tilde{c}_D,\tilde{r}_M\right) & = ∑_{i=1}⁴f_i\left(c_{D,i}-E(\tilde{c}_D)\right)\left(r_{M,i}-E(\tilde{r}_M)\right), \\
      \Cov\left(\tilde{c}_E,\tilde{r}_M\right) & = ∑_{i=1}⁴f_i\left(c_{E,i}-E(\tilde{c}_E)\right)\left(r_{M,i}-E(\tilde{r}_M)\right),
    \end{align}
  \end{subequations}
  \\
  \begin{subequations}
    \begin{align}
      V_D & = \frac{E(\tilde{c}_D) - \left( E\left(\tilde{r}_M\right) - r_f\right)\mfrac{\Cov\left(\tilde{c}_D,\tilde{r}_M\right)}{\sigma_{\tilde{r}_M}²}}{1+r_f}, \\
      V_U & = \frac{E(\tilde{c}_E) - \left( E\left(\tilde{r}_M\right) - r_f\right)\mfrac{\Cov\left(\tilde{c}_E,\tilde{r}_M\right)}{\sigma_{\tilde{r}_M}²}}{1+r_f},
    \end{align}
  \end{subequations}\\
  \begin{subequations}
    \begin{alignat}{2}
        & & \mu_D & = \frac{E(\tilde{c}_D)}{V_D} - 1 \\
      \ArrowBetweenLines[\text{und~}]
        & & \mu_E & = \frac{E(\tilde{c}_E)}{V_E} - 1.
    \end{alignat}
  \end{subequations}
\end{gather}

\end{document}