How many pairs of integers $a$ and $b$ are there such that $a$ and $b$ are between $1$ and $42$ and $a^9 = b^7 \mod 43$?
1. **Understanding the problem**: We need to find the number of pairs of integers \( (a, b) \) such that \( 1 \leq a, b \leq 42 \) and \( a^9 \equiv b^7 \pmod{43} \).

2. **Using Fermat's Little Theorem**: Since \( 43 \) is a prime number, Fermat's Little Theorem tells us that for any integer \( x \) not divisible by \( 43 \), we have \( x^{42} \equiv 1 \pmod{43} \). This implies:
   \[
   b^{42} \equiv 1 \pmod{43}
   \]
   Raising both sides of the given congruence \( a^9 \equiv b^7 \pmod{43} \) to the sixth power, we get:
   \[
   (a^9)^6 \equiv (b^7)^6 \pmod{43} \implies a^{54} \equiv b^{42} \pmod{43}
   \]
   Since \( b^{42} \equiv 1 \pmod{43} \), we have:
   \[
   a^{54} \equiv 1 \pmod{43}
   \]

3. **Simplifying the exponent**: Using Fermat's Little Theorem again, \( a^{42} \equiv 1 \pmod{43} \). Therefore:
   \[
   a^{54} = a^{42} \cdot a^{12} \equiv 1 \cdot a^{12} \equiv a^{12} \pmod{43}
   \]
   Thus, we have:
   \[
   a^{12} \equiv 1 \pmod{43}
   \]

4. **Determining the order of \( a \)**: The order of \( a \) modulo \( 43 \) must divide both \( 42 \) (from Fermat's Little Theorem) and \( 12 \) (from the above congruence). The common divisors of \( 42 \) and \( 12 \) are \( 1, 2, 3, 6 \).

5. **Counting elements of each order**: The number of elements with a specific order \( d \) is given by Euler's totient function \( \phi(d) \):
   - Order \( 1 \): \( \phi(1) = 1 \)
   - Order \( 2 \): \( \phi(2) = 1 \)
   - Order \( 3 \): \( \phi(3) = 2 \)
   - Order \( 6 \): \( \phi(6) = 2 \)

6. **Case 1: \( a \) has order \( 1 \) or \( 3 \)**:
   - If \( a \) has order \( 1 \), \( a^9 \equiv 1 \pmod{43} \).
   - If \( a \) has order \( 3 \), \( a^9 \equiv 1 \pmod{43} \).
   - For \( b \), \( b^7 \equiv 1 \pmod{43} \). The number of such \( b \) is the number of elements of order \( 1 \) or \( 7 \):
     - Order \( 1 \): \( \phi(1) = 1 \)
     - Order \( 7 \): \( \phi(7) = 6 \)
   - Total pairs: \( (1 + 2) \times (1 + 6) = 3 \times 7 = 21 \).

7. **Case 2: \( a \) has order \( 2 \) or \( 6 \)**:
   - If \( a \) has order \( 2 \) or \( 6 \), \( a^9 \equiv -1 \pmod{43} \).
   - For \( b \), \( b^7 \equiv -1 \pmod{43} \). The number of such \( b \) is the number of elements of order \( 14 \) or \( 2 \):
     - Order \( 14 \): \( \phi(14) = 6 \)
     - Order \( 2 \): \( \phi(2) = 1 \)
   - Total pairs: \( (1 + 2) \times (6 + 1) = 3 \times 7 = 21 \).

8. **Summing up the solutions**: The total number of pairs is:
   \[
   21 + 21 = 42
   \]

The final answer is \(\boxed{42}\)